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`.

hermes_cli/runtime_provider.py

@@ -15,12 +15,14 @@ from hermes_cli.auth import (
     AuthError,
     DEFAULT_CODEX_BASE_URL,
     DEFAULT_QWEN_BASE_URL,
+    DEFAULT_XAI_OAUTH_BASE_URL,
     PROVIDER_REGISTRY,
     _agent_key_is_usable,
     format_auth_error,
     resolve_provider,
     resolve_nous_runtime_credentials,
     resolve_codex_runtime_credentials,
+    resolve_xai_oauth_runtime_credentials,
     resolve_qwen_runtime_credentials,
     resolve_gemini_oauth_runtime_credentials,
     resolve_api_key_provider_credentials,
@@ -238,6 +240,9 @@ def _resolve_runtime_from_pool_entry(
     if provider == "openai-codex":
         api_mode = "codex_responses"
         base_url = base_url or DEFAULT_CODEX_BASE_URL
+    elif provider == "xai-oauth":
+        api_mode = "codex_responses"
+        base_url = base_url or DEFAULT_XAI_OAUTH_BASE_URL
     elif provider == "qwen-oauth":
         api_mode = "chat_completions"
         base_url = base_url or DEFAULT_QWEN_BASE_URL
@@ -1132,6 +1137,24 @@ def resolve_runtime_provider(
             logger.info("Auto-detected Codex provider but credentials failed; "
                         "falling through to next provider.")
 
+    if provider == "xai-oauth":
+        try:
+            creds = resolve_xai_oauth_runtime_credentials()
+            return {
+                "provider": "xai-oauth",
+                "api_mode": "codex_responses",
+                "base_url": (creds.get("base_url") or "").rstrip("/") or DEFAULT_XAI_OAUTH_BASE_URL,
+                "api_key": creds.get("api_key", ""),
+                "source": creds.get("source", "hermes-auth-store"),
+                "last_refresh": creds.get("last_refresh"),
+                "requested_provider": requested_provider,
+            }
+        except AuthError:
+            if requested_provider != "auto":
+                raise
+            logger.info("Auto-detected xAI OAuth provider but credentials failed; "
+                        "falling through to next provider.")
+
     if provider == "qwen-oauth":
         try:
             creds = resolve_qwen_runtime_credentials()