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.

providers/gemini.py

@@ -1,72 +0,0 @@
-"""Google Gemini provider profiles.
-
-gemini:            Google AI Studio (API key) — uses GeminiNativeClient
-google-gemini-cli: Google Cloud Code Assist (OAuth) — uses GeminiCloudCodeClient
-
-Both report api_mode="chat_completions" but use custom native clients
-that bypass the standard OpenAI transport. The profile captures auth
-and endpoint metadata for auth.py / runtime_provider.py migration, and
-carries the thinking_config translation hook so the transport's profile
-path produces the same extra_body shape the legacy flag path did.
-"""
-
-from typing import Any
-
-from providers import register_provider
-from providers.base import ProviderProfile
-
-
-class GeminiProfile(ProviderProfile):
-    """Gemini — translate reasoning_config to thinking_config in extra_body."""
-
-    def build_extra_body(
-        self, *, session_id: str | None = None, **context: Any
-    ) -> dict[str, Any]:
-        """Emit extra_body.thinking_config (native) or extra_body.extra_body.google.thinking_config
-        (OpenAI-compat /openai subpath), mirroring the legacy path's behavior.
-        """
-        from agent.transports.chat_completions import (
-            _build_gemini_thinking_config,
-            _is_gemini_openai_compat_base_url,
-            _snake_case_gemini_thinking_config,
-        )
-
-        model = context.get("model") or ""
-        reasoning_config = context.get("reasoning_config")
-        base_url = context.get("base_url") or self.base_url
-
-        raw_thinking_config = _build_gemini_thinking_config(model, reasoning_config)
-        if not raw_thinking_config:
-            return {}
-
-        body: dict[str, Any] = {}
-        if self.name == "gemini" and _is_gemini_openai_compat_base_url(base_url):
-            thinking_config = _snake_case_gemini_thinking_config(raw_thinking_config)
-            if thinking_config:
-                body["extra_body"] = {"google": {"thinking_config": thinking_config}}
-        else:
-            body["thinking_config"] = raw_thinking_config
-        return body
-
-
-gemini = GeminiProfile(
-    name="gemini",
-    aliases=("google", "google-gemini", "google-ai-studio"),
-    api_mode="chat_completions",
-    env_vars=("GOOGLE_API_KEY", "GEMINI_API_KEY"),
-    base_url="https://generativelanguage.googleapis.com/v1beta",
-    auth_type="api_key",
-    default_aux_model="gemini-3-flash-preview",
-)
-
-google_gemini_cli = GeminiProfile(
-    name="google-gemini-cli",
-    aliases=("gemini-cli", "gemini-oauth"),
-    api_mode="chat_completions",
-    env_vars=(),  # OAuth — no API key
-    base_url="cloudcode-pa://google",  # Cloud Code Assist internal scheme
-    auth_type="oauth_external",
-)
-
-register_provider(gemini)
-register_provider(google_gemini_cli)