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.

plugins/model-providers/opencode-zen/__init__.py

@@ -0,0 +1,30 @@
+"""OpenCode provider profiles (Zen + Go).
+
+Both use per-model api_mode routing:
+  - OpenCode Zen: Claude → anthropic_messages, GPT-5/Codex → codex_responses,
+    everything else → chat_completions (this profile)
+  - OpenCode Go: MiniMax → anthropic_messages, GLM/Kimi → chat_completions
+    (this profile)
+"""
+
+from providers import register_provider
+from providers.base import ProviderProfile
+
+opencode_zen = ProviderProfile(
+    name="opencode-zen",
+    aliases=("opencode", "opencode_zen", "zen"),
+    env_vars=("OPENCODE_ZEN_API_KEY",),
+    base_url="https://opencode.ai/zen/v1",
+    default_aux_model="gemini-3-flash",
+)
+
+opencode_go = ProviderProfile(
+    name="opencode-go",
+    aliases=("opencode_go", "go", "opencode-go-sub"),
+    env_vars=("OPENCODE_GO_API_KEY",),
+    base_url="https://opencode.ai/zen/go/v1",
+    default_aux_model="glm-5",
+)
+
+register_provider(opencode_zen)
+register_provider(opencode_go)