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/tools_config.py

@@ -1753,6 +1753,62 @@ def _plugin_browser_providers() -> list[dict]:
     return rows
 
 
+def _plugin_tts_providers() -> list[dict]:
+    """Build picker-row dicts from plugin-registered TTS providers.
+
+    Issue #30398 — the ``register_tts_provider()`` plugin hook
+    coexists alongside the 10 built-in TTS providers
+    (``edge``/``openai``/``elevenlabs``/…) and the
+    ``tts.providers.<name>: type: command`` registry from PR #17843.
+    Built-in rows stay hardcoded in ``TOOL_CATEGORIES["tts"]``; this
+    function only injects PLUGIN-registered providers.
+
+    Defensive: plugins whose name collides with a built-in TTS provider
+    are filtered out — even though the registry already rejects them
+    at registration time, a future code path that registers directly
+    via :func:`agent.tts_registry.register_provider` could slip
+    through. Filtering here keeps the picker invariant.
+    """
+    try:
+        from agent.tts_registry import _BUILTIN_NAMES, list_providers
+        from hermes_cli.plugins import _ensure_plugins_discovered
+
+        _ensure_plugins_discovered()
+        providers = list_providers()
+    except Exception:
+        return []
+
+    rows: list[dict] = []
+    for provider in providers:
+        name = getattr(provider, "name", None)
+        if not name:
+            continue
+        # Defensive: reject built-in shadowing at the picker layer too.
+        if name.lower().strip() in _BUILTIN_NAMES:
+            continue
+        try:
+            schema = provider.get_setup_schema()
+        except Exception:
+            continue
+        if not isinstance(schema, dict):
+            continue
+        row = {
+            "name": schema.get("name", provider.display_name),
+            "badge": schema.get("badge", ""),
+            "tag": schema.get("tag", ""),
+            "env_vars": schema.get("env_vars", []),
+            # Selecting this row writes ``tts.provider: <name>`` — the
+            # same write-path used by hardcoded rows. The plugin
+            # dispatcher picks it up automatically from there.
+            "tts_provider": name,
+            "tts_plugin_name": name,
+        }
+        if schema.get("post_setup"):
+            row["post_setup"] = schema["post_setup"]
+        rows.append(row)
+    return rows
+
+
 def _visible_providers(cat: dict, config: dict) -> list[dict]:
     """Return provider entries visible for the current auth/config state."""
     features = get_nous_subscription_features(config)
@@ -1790,6 +1846,12 @@ def _visible_providers(cat: dict, config: dict) -> list[dict]:
     if cat.get("name") == "Browser Automation":
         visible.extend(_plugin_browser_providers())
 
+    # Inject plugin-registered TTS backends (issue #30398). Plugin rows
+    # render BELOW the 10 hardcoded built-in rows. Built-in shadowing
+    # is filtered out by ``_plugin_tts_providers`` defensively.
+    if cat.get("name") == "Text-to-Speech":
+        visible.extend(_plugin_tts_providers())
+
     return visible