feat(stt): add register_transcription_provider() plugin hook

Add an opt-in Python plugin surface for speech-to-text backends,
mirroring the TTS hook pattern. New backends (OpenRouter, SenseAudio,
Gemini-STT, custom proprietary engines) can be implemented as plugins
without modifying tools/transcription_tools.py.

Built-ins always win
--------------------
The 6 built-in STT providers (local/faster-whisper, local_command,
groq, openai, mistral, xai) keep their native handlers. Plugins
attempting to register under a built-in name are rejected at
registration time with a warning and re-checked defensively at
dispatch.

Resolution order
----------------
1. stt.provider matches a built-in → built-in dispatch (unchanged)
2. stt.provider matches a registered plugin →
   a. if plugin.is_available() returns False → unavailability envelope
      identifying the plugin (not the generic "No STT provider"
      message — the user explicitly opted into this plugin)
   b. otherwise plugin.transcribe() with model + language forwarded
      from stt.<provider>.{model,language} config
3. No match → legacy "No STT provider available" error (unchanged)

Per-provider config namespace
-----------------------------
Plugins read their config from stt.<provider> in config.yaml, mirroring
how built-ins read stt.openai.model / stt.mistral.model. The dispatcher
forwards `model` and `language` from this section. Caller's explicit
`model=` argument overrides the config-set model.

Files
-----
- agent/transcription_provider.py: TranscriptionProvider ABC
- agent/transcription_registry.py: register/get/list providers,
  built-in shadow guard, _reset_for_tests
- hermes_cli/plugins.py: register_transcription_provider() on
  PluginContext
- tools/transcription_tools.py: BUILTIN_STT_PROVIDERS frozenset,
  _dispatch_to_plugin_provider() with availability gate, wire-in
  after xai branch and before "No STT provider" error
- tests/agent/test_transcription_registry.py: 27 tests
- tests/hermes_cli/test_plugins_transcription_registration.py: 3 tests
- tests/tools/test_transcription_plugin_dispatch.py: 28 tests
  (covering built-in short-circuit, plugin dispatch, exception
  envelope, non-dict guard, availability gate, language forwarding)
- tests/plugins/transcription/check_parity_vs_main.py: 10-scenario
  subprocess-pinned parity harness vs origin/main
- website/docs/user-guide/features/{tts,plugins}.md: docs

Behavior parity
---------------
10 scenarios, 8 OK + 2 expected DIFFs:
  no_provider_error → plugin (plugin-installed scenario)
  no_provider_error → plugin_unavailable (plugin-installed-unavailable
  scenario; PR returns cleaner envelope)
Zero behavior change for users not opting into a plugin.

Issue follow-up to #30398.

hermes_cli/plugins.py

@@ -678,6 +678,50 @@ class PluginContext:
             self.manifest.name, provider.name,
         )
 
+    # -- transcription (STT) provider registration ---------------------------
+
+    def register_transcription_provider(self, provider) -> None:
+        """Register a speech-to-text backend.
+
+        ``provider`` must be an instance of
+        :class:`agent.transcription_provider.TranscriptionProvider`.
+        The ``provider.name`` attribute is what ``stt.provider`` in
+        ``config.yaml`` matches against when routing
+        :func:`tools.transcription_tools.transcribe_audio` calls —
+        **but only when**:
+
+        1. ``provider.name`` is NOT a built-in STT provider name
+           (``local``, ``local_command``, ``groq``, ``openai``,
+           ``mistral``, ``xai``). Built-ins always win — the registry
+           rejects shadowing names with a warning.
+        2. There is NO ``stt.providers.<name>: type: command`` entry
+           with the same name. Command-providers win on name
+           collision because config is more local than plugin install
+           — same precedence rule as TTS.
+
+        Coexists with the in-tree dispatcher and the STT
+        command-provider registry rather than replacing them. The 6
+        built-in STT backends keep their native implementations in
+        ``tools/transcription_tools.py``; this hook is for *new* Python
+        engines (OpenRouter, SenseAudio, Gemini-STT, custom proprietary
+        backends).
+        """
+        from agent.transcription_provider import TranscriptionProvider
+        from agent.transcription_registry import register_provider as _register_stt_provider
+
+        if not isinstance(provider, TranscriptionProvider):
+            logger.warning(
+                "Plugin '%s' tried to register a transcription provider that "
+                "does not inherit from TranscriptionProvider. Ignoring.",
+                self.manifest.name,
+            )
+            return
+        _register_stt_provider(provider)
+        logger.info(
+            "Plugin '%s' registered transcription provider: %s",
+            self.manifest.name, provider.name,
+        )
+
     # -- platform adapter registration ---------------------------------------
 
     def register_platform(