docs: pluggable surfaces coverage — model-provider guide, full plugin map, opt-in fix (#20749)

* docs(providers): add model-provider-plugin authoring guide + fix stale refs

New docs:
- website/docs/developer-guide/model-provider-plugin.md — full authoring
  guide (directory layout, minimal example, ProviderProfile fields,
  overridable hooks, user overrides, api_mode selection, auth types,
  testing, pip distribution)
- Wired into website/sidebars.ts under 'Extending'
- Cross-references added in:
  - guides/build-a-hermes-plugin.md (tip block)
  - developer-guide/adding-providers.md
  - developer-guide/provider-runtime.md

User guide:
- user-guide/features/plugins.md: Plugin types table grows from 3 to 4
  with 'Model providers' row

Stale comment cleanup (providers/*.py → plugins/model-providers/<name>/):
- hermes_cli/main.py:_is_profile_api_key_provider docstring
- hermes_cli/doctor.py:_build_apikey_providers_list docstring
- hermes_cli/auth.py: PROVIDER_REGISTRY + alias auto-extension comments
- hermes_cli/models.py: CANONICAL_PROVIDERS auto-extension comment

AGENTS.md:
- Project-structure tree: added plugins/model-providers/ row
- New section: 'Model-provider plugins' explaining discovery, override
  semantics, PluginManager integration, kind auto-coerce heuristic

Verified: docusaurus build succeeds, new page renders, all 3 cross-links
resolve. 347/347 targeted tests pass (tests/providers/,
tests/hermes_cli/test_plugins.py, tests/hermes_cli/test_runtime_provider_resolution.py,
tests/run_agent/test_provider_parity.py).

* docs(plugins): add 'pluggable interfaces at a glance' maps to plugins.md + build-a-hermes-plugin

Devs landing on either the user-guide plugin page or the build-a-plugin
guide now get an upfront table of every distinct pluggable surface with
a link to the right authoring doc. Previously they'd have to read the
full general-plugin guide to discover that model providers / platforms
/ memory / context engines are separate systems.

user-guide/features/plugins.md:
- New 'Pluggable interfaces — where to go for each' section below the
  existing 4-kinds table
- 10 rows covering every register_* surface (tool, hook, slash command,
  CLI subcommand, skill, model provider, platform, memory, context
  engine, image-gen)
- Explicit note: TTS/STT are NOT plugin-extensible yet — documented
  with a pointer to the current config.yaml 'command providers' pattern
  and a note that register_tts_provider()/register_stt_provider() may
  come later

guides/build-a-hermes-plugin.md:
- New :::info 'Not sure which guide you need?' map at the top so devs
  see all pluggable interfaces before investing in this 737-line
  general-plugin walkthrough
- Existing bottom :::tip expanded to include platform adapters alongside
  model/memory/context plugins

Verified:
- All 8 cross-doc links in the new plugins.md table resolve in a
  docusaurus build (SUCCESS, no new broken links)
- TTS link corrected (features/voice → features/tts; latter exists)
- Pre-existing broken links/anchors (cron-script-only, llms.txt,
  adding-platform-adapters#step-by-step-checklist) are unchanged

* docs(plugins): correct TTS/STT pluggability \u2014 they ARE plugins (command-providers)

Previous commit incorrectly said TTS/STT 'aren't plugin-extensible'. They
are, via the config-driven command-provider pattern \u2014 any CLI that reads
text and writes audio (or vice versa for STT) is automatically a plugin
with zero Python. The tts.md docs cover this extensively and I missed it.

plugins.md:
- TTS row: 'Config-driven (not a Python plugin)', points at
  tts.md#custom-command-providers
- STT row: points at tts.md#voice-message-transcription-stt (STT docs
  live in tts.md despite the filename)
- Expanded note: TTS/STT use config-driven shell-command templates as
  their plugin surface (full tts.providers.<name> registry for TTS;
  HERMES_LOCAL_STT_COMMAND escape hatch for STT)
- Any CLI that reads/writes files is automatically a plugin \u2014 no Python
  register_* API needed
- Future register_tts_provider()/register_stt_provider() hooks mentioned
  as nice-to-have for SDK/streaming cases, not as the primary story

build-a-hermes-plugin.md:
- Same map update: TTS/STT rows explicit, footer note corrected

Verified:
- tts.md anchors (custom-command-providers, voice-message-transcription-stt)
  exist and resolve in docusaurus build (SUCCESS, no new broken links)

* docs(plugins): expand pluggable interfaces table with MCP / event hooks / shell hooks / skill taps

Broadened the scope beyond Python register_* hooks. Hermes has MULTIPLE
plugin-style extension surfaces; they're now all in one table instead of
being scattered across feature docs.

Added rows for:
- **MCP servers** — config.yaml mcp_servers.<name> auto-registers external
  tools from any MCP server. Huge extensibility surface, previously not
  linked from the plugin map.
- **Gateway event hooks** — drop HOOK.yaml + handler.py into
  ~/.hermes/hooks/<name>/ to fire on gateway:startup, session:*, agent:*,
  command:* events. Separate from Python plugin hooks.
- **Shell hooks** — hooks: block in config.yaml runs shell commands on
  events (notifications, auditing, etc.).
- **Skill sources (taps)** — hermes skills tap add <repo> to pull in new
  skill registries beyond the built-in sources.

Both docs updated:
- user-guide/features/plugins.md: table column renamed to 'How' (mixes
  Python API + config-driven + drop-in-dir surfaces accurately)
- guides/build-a-hermes-plugin.md: :::info map at top mirrors the new
  surfaces with a forward-link to the consolidated table

Note block rewritten: instead of singling out TTS/STT as the 'different
style' exception, now honestly describes that Hermes deliberately
supports three plugin styles — Python APIs, config-driven commands, and
drop-in manifest directories — and devs should pick the one that fits
their integration.

Not included (considered and rejected):
- Transport layer (register_transport) — internal, not user-facing
- Tool-call parsers — internal, VLLM phase-2 thing
- Cloud browser providers — hardcoded registry, not drop-in yet
- Terminal backends — hardcoded if/elif, not drop-in yet
- Skill sources (the ABC) — hardcoded list, only taps are user-extensible

Verified:
- All 5 new anchors resolve (gateway-event-hooks, shell-hooks, skills-hub,
  custom-command-providers, voice-message-transcription-stt)
- Docusaurus build SUCCESS, zero new broken links
- Same 3 pre-existing broken links on main (cron-script-only, llms.txt,
  adding-platform-adapters#step-by-step-checklist)

* docs(plugins): cover every pluggable surface in both the overview and how-to

Both plugins.md and build-a-hermes-plugin.md now cover every extension
surface end-to-end \u2014 general plugin APIs, specialized plugin types,
config-driven surfaces \u2014 with concrete authoring patterns for each.

plugins.md:
- 'What plugins can do' table grows from 9 rows (general ctx.register_*
  only) to 14 rows covering register_platform, register_image_gen_provider,
  register_context_engine, MemoryProvider subclass, register_provider
  (model). Each row links to its full authoring guide.
- New 'Plugin sub-categories' section under Plugin Discovery explains
  how plugins/platforms/, plugins/image_gen/, plugins/memory/,
  plugins/context_engine/, plugins/model-providers/ are routed to
  different loaders \u2014 PluginManager vs the per-category own-loader
  systems.
- Explicit mention of user-override semantics at
  ~/.hermes/plugins/model-providers/ and ~/.hermes/plugins/memory/.

build-a-hermes-plugin.md:
- New '## Specialized plugin types' section (5 sub-sections):
  - Model provider plugins \u2014 ProviderProfile + plugin.yaml example,
    auto-wiring summary, link to full guide
  - Platform plugins \u2014 BasePlatformAdapter + register_platform() skeleton
  - Memory provider plugins \u2014 MemoryProvider subclass example
  - Context engine plugins \u2014 ContextEngine subclass example
  - Image-generation backends \u2014 ImageGenProvider + kind: backend example
- New '## Non-Python extension surfaces' section (5 sub-sections):
  - MCP servers \u2014 config.yaml mcp_servers.<name> example
  - Gateway event hooks \u2014 HOOK.yaml + handler.py example
  - Shell hooks \u2014 hooks: block in config.yaml example
  - Skill sources (taps) \u2014 hermes skills tap add example
  - TTS / STT command templates \u2014 tts.providers.<name> with type: command
- Distribute via pip / NixOS promoted from ### to ## (they were orphaned
  after the reorganization)

Each specialized / non-Python section has a concrete, copy-pasteable
example plus a 'Full guide:' link to the authoritative doc. Devs arriving
at the build-a-hermes-plugin guide now see every extension surface at
their disposal, not just the general tool/hook/slash-command surface.

Verified:
- Docusaurus build SUCCESS, zero new broken links
- All new cross-links (developer-guide/model-provider-plugin,
  adding-platform-adapters, memory-provider-plugin, context-engine-plugin,
  user-guide/features/mcp, skills#skills-hub, hooks#gateway-event-hooks,
  hooks#shell-hooks, tts#custom-command-providers,
  tts#voice-message-transcription-stt) resolve
- Same 3 pre-existing broken links on main (cron-script-only, llms.txt,
  adding-platform-adapters#step-by-step-checklist)

* docs(plugins): fix opt-in inconsistency — not every plugin is gated

The 'Every plugin is disabled by default' statement was wrong. Several
plugin categories intentionally bypass plugins.enabled:

- Bundled platform plugins (IRC, Teams) auto-load so shipped gateway
  channels are available out of the box. Activation per channel is via
  gateway.platforms.<name>.enabled.
- Bundled backends (plugins/image_gen/*) auto-load so the default
  backend 'just works'. Selection via <category>.provider config.
- Memory providers are all discovered; one is active via memory.provider.
- Context engines are all discovered; one is active via context.engine.
- Model providers: all 33 discovered at first get_provider_profile();
  user picks via --provider / config.

The plugins.enabled allow-list specifically gates:
- Standalone plugins (general tools/hooks/slash commands)
- User-installed backends
- User-installed platforms (third-party gateway adapters)
- Pip entry-point backends

Which matches the actual code in hermes_cli/plugins.py:737 where the
bundled+backend/platform check bypasses the allow-list.

Rewrote '## Plugins are opt-in' to:
- Retitle to 'Plugins are opt-in (with a few exceptions)'
- Narrow opening claim to 'General plugins and user-installed backends
  are disabled by default'
- Added 'What the allow-list does NOT gate' subsection with a full
  table of which bypass the gate and how they're activated instead
- Fixed migration section wording (bundled platform/backend plugins
  never needed grandfathering)

Verified: docusaurus build SUCCESS, zero new broken links.

website/docs/developer-guide/adding-providers.md

@@ -121,7 +121,7 @@ When you add a plugin and it calls `register_provider()`, the following wire up
 
 User plugins at `$HERMES_HOME/plugins/model-providers/<name>/` override bundled plugins of the same name (last-writer-wins in `register_provider()`) — so third parties can monkey-patch or replace any built-in profile without editing the repo.
 
-See `plugins/model-providers/nvidia/` or `plugins/model-providers/gmi/` as a template, and `plugins/model-providers/README.md` for the full contract.
+See `plugins/model-providers/nvidia/` or `plugins/model-providers/gmi/` as a template, and the full [Model Provider Plugin guide](/docs/developer-guide/model-provider-plugin) for field reference, hook idioms, and end-to-end examples.
 
 ## Full path: OAuth and complex providers