feat(xai-oauth): add xAI Grok OAuth (SuperGrok Subscription) provider

Adds a new authentication provider that lets SuperGrok subscribers sign
in to Hermes with their xAI account via the standard OAuth 2.0 PKCE
loopback flow, instead of pasting a raw API key from console.x.ai.

Highlights
----------
* OAuth 2.0 PKCE loopback login against accounts.x.ai with discovery,
  state/nonce, and a strict CORS-origin allowlist on the callback.
* Authorize URL carries `plan=generic` (required for non-allowlisted
  loopback clients) and `referrer=hermes-agent` for best-effort
  attribution in xAI's OAuth server logs.
* Token storage in `auth.json` with file-locked atomic writes; JWT
  `exp`-based expiry detection with skew; refresh-token rotation
  synced both ways between the singleton store and the credential
  pool so multi-process / multi-profile setups don't tear each other's
  refresh tokens.
* Reactive 401 retry: on a 401 from the xAI Responses API, the agent
  refreshes the token, swaps it back into `self.api_key`, and retries
  the call once. Guarded against silent account swaps when the active
  key was sourced from a different (manual) pool entry.
* Auxiliary tasks (curator, vision, embeddings, etc.) route through a
  dedicated xAI Responses-mode auxiliary client instead of falling back
  to OpenRouter billing.
* Direct HTTP tools (`tools/xai_http.py`, transcription, TTS, image-gen
  plugin) resolve credentials through a unified runtime → singleton →
  env-var fallback chain so xai-oauth users get them for free.
* `hermes auth add xai-oauth` and `hermes auth remove xai-oauth N` are
  wired through the standard auth-commands surface; remove cleans up
  the singleton loopback_pkce entry so it doesn't silently reinstate.
* `hermes model` provider picker shows
  "xAI Grok OAuth (SuperGrok Subscription)" and the model-flow falls
  back to pool credentials when the singleton is missing.

Hardening
---------
* Discovery and refresh responses validate the returned
  `token_endpoint` host against the same `*.x.ai` allowlist as the
  authorization endpoint, blocking MITM persistence of a hostile
  endpoint.
* Discovery / refresh / token-exchange `response.json()` calls are
  wrapped to raise typed `AuthError` on malformed bodies (captive
  portals, proxy error pages) instead of leaking JSONDecodeError
  tracebacks.
* `prompt_cache_key` is routed through `extra_body` on the codex
  transport (sending it as a top-level kwarg trips xAI's SDK with a
  TypeError).
* Credential-pool sync-back preserves `active_provider` so refreshing
  an OAuth entry doesn't silently flip the active provider out from
  under the running agent.

Testing
-------
* New `tests/hermes_cli/test_auth_xai_oauth_provider.py` (~63 tests)
  covers JWT expiry, OAuth URL params (plan + referrer), CORS origins,
  redirect URI validation, singleton↔pool sync, concurrency races,
  refresh error paths, runtime resolution, and malformed-JSON guards.
* Extended `test_credential_pool.py`, `test_codex_transport.py`, and
  `test_run_agent_codex_responses.py` cover the pool sync-back,
  `extra_body` routing, and 401 reactive refresh paths.
* 165 tests passing on this branch via `scripts/run_tests.sh`.

tools/tts_tool.py

@@ -9,7 +9,7 @@ Built-in TTS providers:
 - MiniMax TTS: High-quality with voice cloning, needs MINIMAX_API_KEY
 - Mistral (Voxtral TTS): Multilingual, native Opus, needs MISTRAL_API_KEY
 - Google Gemini TTS: Controllable, 30 prebuilt voices, needs GEMINI_API_KEY
-- xAI TTS: Grok voices, needs XAI_API_KEY
+- xAI TTS: Grok voices, uses xAI Grok OAuth credentials or XAI_API_KEY
 - NeuTTS (local, free, no API key): On-device TTS via neutts
 - KittenTTS (local, free, no API key): On-device 25MB model
 - Piper (local, free, no API key): OHF-Voice/piper1-gpl neural VITS, 44 languages
@@ -902,9 +902,12 @@ def _generate_xai_tts(text: str, output_path: str, tts_config: Dict[str, Any]) -
     """
     import requests
 
-    api_key = (get_env_value("XAI_API_KEY") or "").strip()
+    from tools.xai_http import resolve_xai_http_credentials
+
+    creds = resolve_xai_http_credentials()
+    api_key = str(creds.get("api_key") or "").strip()
     if not api_key:
-        raise ValueError("XAI_API_KEY not set. Get one at https://console.x.ai/")
+        raise ValueError("No xAI credentials found. Configure xAI OAuth in `hermes model` or set XAI_API_KEY.")
 
     xai_config = tts_config.get("xai", {})
     voice_id = str(xai_config.get("voice_id", DEFAULT_XAI_VOICE_ID)).strip() or DEFAULT_XAI_VOICE_ID
@@ -913,6 +916,7 @@ def _generate_xai_tts(text: str, output_path: str, tts_config: Dict[str, Any]) -
     bit_rate = int(xai_config.get("bit_rate", DEFAULT_XAI_BIT_RATE))
     base_url = str(
         xai_config.get("base_url")
+        or creds.get("base_url")
         or get_env_value("XAI_BASE_URL")
         or DEFAULT_XAI_BASE_URL
     ).strip().rstrip("/")
@@ -1917,8 +1921,13 @@ def check_tts_requirements() -> bool:
         pass
     if get_env_value("MINIMAX_API_KEY"):
         return True
-    if get_env_value("XAI_API_KEY"):
-        return True
+    try:
+        from tools.xai_http import resolve_xai_http_credentials
+
+        if resolve_xai_http_credentials().get("api_key"):
+            return True
+    except Exception:
+        pass
     if get_env_value("GEMINI_API_KEY") or get_env_value("GOOGLE_API_KEY"):
         return True
     try: