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/transcription_tools.py

@@ -266,10 +266,12 @@ def _get_provider(stt_config: dict) -> str:
             return "none"
 
         if provider == "xai":
-            if get_env_value("XAI_API_KEY"):
+            from tools.xai_http import resolve_xai_http_credentials
+
+            if resolve_xai_http_credentials().get("api_key"):
                 return "xai"
             logger.warning(
-                "STT provider 'xai' configured but XAI_API_KEY not set"
+                "STT provider 'xai' configured but no xAI credentials are available"
             )
             return "none"
 
@@ -289,9 +291,14 @@ def _get_provider(stt_config: dict) -> str:
     if _HAS_OPENAI and _has_openai_audio_backend():
         logger.info("No local STT available, using OpenAI Whisper API")
         return "openai"
-    if get_env_value("XAI_API_KEY"):
-        logger.info("No local STT available, using xAI Grok STT API")
-        return "xai"
+    try:
+        from tools.xai_http import resolve_xai_http_credentials
+
+        if resolve_xai_http_credentials().get("api_key"):
+            logger.info("No local STT available, using xAI Grok STT API")
+            return "xai"
+    except Exception:
+        pass
     return "none"
 
 # ---------------------------------------------------------------------------
@@ -704,14 +711,22 @@ def _transcribe_xai(file_path: str, model_name: str) -> Dict[str, Any]:
     Supports Inverse Text Normalization, diarization, and word-level timestamps.
     Requires ``XAI_API_KEY`` environment variable.
     """
-    api_key = get_env_value("XAI_API_KEY")
+    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:
-        return {"success": False, "transcript": "", "error": "XAI_API_KEY not set"}
+        return {
+            "success": False,
+            "transcript": "",
+            "error": "No xAI credentials found. Configure xAI OAuth in `hermes model` or set XAI_API_KEY",
+        }
 
     stt_config = _load_stt_config()
     xai_config = stt_config.get("xai", {})
     base_url = str(
         xai_config.get("base_url")
+        or creds.get("base_url")
         or get_env_value("XAI_STT_BASE_URL")
         or XAI_STT_BASE_URL
     ).strip().rstrip("/")
@@ -872,7 +887,7 @@ def transcribe_audio(file_path: str, model: Optional[str] = None) -> Dict[str, A
             "No STT provider available. Install faster-whisper for free local "
             f"transcription, configure {LOCAL_STT_COMMAND_ENV} or install a local whisper CLI, "
             "set GROQ_API_KEY for free Groq Whisper, set MISTRAL_API_KEY for Mistral "
-            "Voxtral Transcribe, set XAI_API_KEY for xAI Grok STT, or set VOICE_TOOLS_OPENAI_KEY "
+            "Voxtral Transcribe, configure xAI OAuth or set XAI_API_KEY for xAI Grok STT, or set VOICE_TOOLS_OPENAI_KEY "
             "or OPENAI_API_KEY for the OpenAI Whisper API."
         ),
     }