feat(providers): make all 33 providers pluggable under plugins/model-providers/

Every provider profile is now a self-contained plugin under
plugins/model-providers/<name>/, mirroring the plugins/platforms/
pattern established for IRC and Teams. The ProviderProfile ABC
stays in providers/; the per-provider profile data moves out.

- plugins/model-providers/<name>/__init__.py calls register_provider()
- plugins/model-providers/<name>/plugin.yaml declares kind: model-provider
- providers/__init__.py._discover_providers() lazily scans bundled plugins
  then $HERMES_HOME/plugins/model-providers/<name>/ (user override path)
- User plugins with the same name override bundled ones (last-writer-wins
  in register_provider)
- Legacy providers/<name>.py layout still supported for back-compat with
  out-of-tree editable installs
- Hermes PluginManager: new kind=model-provider; skipped like memory
  plugins (providers/ discovery owns them); standalone plugins with
  register_provider+ProviderProfile in their __init__.py auto-coerce to
  this kind (same heuristic as memory providers)
- skip_names extended to include 'model-providers' so the general
  PluginManager doesn't double-scan the category
- 4 new tests in tests/providers/test_plugin_discovery.py covering
  bundled discovery, user override, and general-loader isolation
- Docs updated: website/docs/developer-guide/adding-providers.md,
  provider-runtime.md, providers/README.md, plugins/model-providers/README.md

No API break: auth.py / config.py / doctor.py / models.py / runtime_provider.py /
model_metadata.py / auxiliary_client.py / chat_completions.py / run_agent.py
all still consume providers via get_provider_profile() / list_providers() —
they just now see plugin-discovered entries instead of pkgutil-iterated ones.

Third parties can now drop a single directory into
~/.hermes/plugins/model-providers/<name>/ to add or override an inference
provider without touching the repo.

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

@@ -99,10 +99,12 @@ If your provider is just an OpenAI-compatible endpoint that authenticates with a
 
 All you need is:
 
-1. A file in `providers/` (e.g. `providers/myprovider.py`) that calls `register_provider()` with the provider config.
-2. That's it. `auth.py` auto-registers every file in `providers/` at startup via a module-level import sweep.
+1. A plugin directory under `plugins/model-providers/<your-provider>/` containing:
+   - `__init__.py` — calls `register_provider(profile)` at module-level
+   - `plugin.yaml` — manifest (name, kind: model-provider, version, description)
+2. That's it. Provider plugins auto-load the first time anything calls `get_provider_profile()` or `list_providers()` — bundled plugins (this repo) and user plugins at `$HERMES_HOME/plugins/model-providers/` both get picked up.
 
-When you add a `providers/*.py` file and call `register_provider()`, the following wire up automatically:
+When you add a plugin and it calls `register_provider()`, the following wire up automatically:
 
 1. `PROVIDER_REGISTRY` entry in `auth.py` (credential resolution, env-var lookup)
 2. `api_mode` set to `chat_completions`
@@ -117,7 +119,9 @@ When you add a `providers/*.py` file and call `register_provider()`, the followi
 11. `HERMES_INFERENCE_PROVIDER` env-var override accepts the provider id
 12. Fallback model activation can switch into the provider cleanly
 
-See `providers/nvidia.py` or `providers/gmi.py` as a template.
+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.
 
 ## Full path: OAuth and complex providers