feat(tts): add register_tts_provider() plugin hook (closes #30398)

Adds a `TTSProvider(ABC)` + `register_tts_provider()` extension point
to the plugin context API, **alongside** the existing config-driven
`tts.providers.<name>: type: command` registry from PR #17843. This is
additive — the command-provider surface stays as the primary way to
add a TTS backend.

The hook covers cases the shell-template grammar can't reasonably
express:

- Native Python SDKs without a CLI (Cartesia, Fish Audio, etc.)
- Streaming synthesis (chunked Opus → voice-bubble delivery)
- Voice metadata API for the `hermes tools` picker
- OAuth-refreshing auth flows

None of the 10 inline built-in providers (`edge`, `openai`,
`elevenlabs`, `minimax`, `gemini`, `mistral`, `xai`, `piper`,
`kittentts`, `neutts`) are migrated to plugins. They stay inline. The
hook is for *new* engines that aren't built-in.

## Resolution order

The dispatcher's resolution order is the load-bearing invariant:

1. `tts.provider` is a built-in name → built-in dispatch. **Always wins.**
2. `tts.provider` matches `tts.providers.<name>` with `command:` set
   → command-provider dispatch (PR #17843).
3. `tts.provider` matches a plugin-registered `TTSProvider`
   → plugin dispatch (new).
4. No match → falls through to Edge TTS default (legacy behavior).

Built-ins-always-win is enforced at THREE layers:
- Registry: `register_provider()` rejects shadowing names with a warning.
- Dispatcher: `_dispatch_to_plugin_provider()` short-circuits built-in
  names defensively before consulting the registry.
- Picker: `_plugin_tts_providers()` filters built-in shadows out of
  the `hermes tools` row list defensively.

Command-providers-win-over-plugins is enforced at TWO layers:
- The caller in `text_to_speech_tool` checks
  `_resolve_command_provider_config` first.
- `_dispatch_to_plugin_provider` re-checks for a same-name command
  config defensively so a refactor of the caller can't silently break
  the invariant.

## New files

- `agent/tts_provider.py` — `TTSProvider(ABC)` with `synthesize()` (required),
  `list_voices()`, `list_models()`, `get_setup_schema()`, `stream()`,
  `voice_compatible` (all optional with sane defaults). Mirrors
  `agent/image_gen_provider.py` shape.
- `agent/tts_registry.py` — `register_provider`/`get_provider`/`list_providers`
  with `_BUILTIN_NAMES` reject-shadowing invariant. Mirrors
  `agent/image_gen_registry.py` shape.
- `plugins/tts/...` directory ready for community plugins (none shipped).

## Modified files

- `hermes_cli/plugins.py` — `register_tts_provider()` method on
  `PluginContext`. Matches the gating shape of
  `register_image_gen_provider()` / `register_browser_provider()`.
- `tools/tts_tool.py` — `_dispatch_to_plugin_provider()` +
  `_plugin_provider_is_voice_compatible()` + walrus-elif wiring into
  the main dispatcher. Built-in elif chain untouched.
- `hermes_cli/tools_config.py` — `_plugin_tts_providers()` injects
  plugin rows into the Text-to-Speech picker category alongside the
  10 hardcoded built-in rows.

## Tests

- `tests/agent/test_tts_registry.py` — 47 tests covering registration,
  lookup, ABC contract, helpers, AND a `TestBuiltinSync` regression
  test that fails if `agent.tts_registry._BUILTIN_NAMES` drifts from
  `tools.tts_tool.BUILTIN_TTS_PROVIDERS` (kept duplicated due to
  circular import constraints).
- `tests/tools/test_tts_plugin_dispatch.py` — 35 tests covering
  built-in-always-wins, command-wins-over-plugin, plugin dispatch,
  exception passthrough, voice_compatible helper.
- `tests/hermes_cli/test_tts_picker.py` — 10 tests covering the
  picker surface, builtin shadowing defense, integration with
  `_visible_providers`.
- `tests/hermes_cli/test_plugins_tts_registration.py` — 3 end-to-end
  tests via `PluginManager.discover_and_load()`.
- `tests/plugins/tts/check_parity_vs_main.py` — 9-scenario subprocess
  parity harness vs `origin/main`. The only intentional diff is
  `fallback_edge → plugin` for the `plugin-installed` scenario.

## Verification

- 95/95 new tests pass.
- 170/170 pre-existing TTS tests (test_tts_command_providers,
  test_tts_max_text_length, test_tts_speed, etc.) pass unchanged.
- Parity harness against `origin/main`: 8 OK + 1 expected DIFF.
- E2E smoke: a registered plugin's `synthesize()` is called via
  `text_to_speech_tool` with the standard JSON envelope returned.
- Ruff clean on all touched files.

## Docs

- `website/docs/user-guide/features/tts.md` — new "Python plugin
  providers" section with a decision table (command-provider vs
  plugin), minimal plugin example, and the optional-hook reference.
- `website/docs/user-guide/features/plugins.md` — TTS row updated to
  mention both surfaces (command-provider primary, plugin for
  SDK/streaming).

Closes #30398

hermes_cli/plugins.py

@@ -640,6 +640,44 @@ class PluginContext:
             self.manifest.name, provider.name,
         )
 
+    # -- TTS provider registration -------------------------------------------
+
+    def register_tts_provider(self, provider) -> None:
+        """Register a text-to-speech backend.
+
+        ``provider`` must be an instance of
+        :class:`agent.tts_provider.TTSProvider`. The ``provider.name``
+        attribute is what ``tts.provider`` in ``config.yaml`` matches
+        against when routing ``text_to_speech`` tool calls — **but
+        only when**:
+
+        1. ``provider.name`` is NOT a built-in TTS provider name
+           (``edge``, ``openai``, ``elevenlabs``, …). Built-ins always
+           win — the registry rejects shadowing names with a warning.
+        2. There is NO ``tts.providers.<name>: type: command`` entry
+           with the same name. Command-providers (PR #17843) win on
+           name collision because config is more local than plugin
+           install.
+
+        Coexists with the command-provider registry rather than
+        replacing it — see issue #30398 for the full design rationale.
+        """
+        from agent.tts_provider import TTSProvider
+        from agent.tts_registry import register_provider as _register_tts_provider
+
+        if not isinstance(provider, TTSProvider):
+            logger.warning(
+                "Plugin '%s' tried to register a TTS provider that does "
+                "not inherit from TTSProvider. Ignoring.",
+                self.manifest.name,
+            )
+            return
+        _register_tts_provider(provider)
+        logger.info(
+            "Plugin '%s' registered TTS provider: %s",
+            self.manifest.name, provider.name,
+        )
+
     # -- platform adapter registration ---------------------------------------
 
     def register_platform(