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`.

agent/codex_responses_adapter.py

@@ -726,7 +726,7 @@ def _preflight_codex_api_kwargs(
         "model", "instructions", "input", "tools", "store",
         "reasoning", "include", "max_output_tokens", "temperature",
         "tool_choice", "parallel_tool_calls", "prompt_cache_key", "service_tier",
-        "extra_headers",
+        "extra_headers", "extra_body",
     }
     normalized: Dict[str, Any] = {
         "model": model,
@@ -776,6 +776,19 @@ def _preflight_codex_api_kwargs(
         if normalized_headers:
             normalized["extra_headers"] = normalized_headers
 
+    extra_body = api_kwargs.get("extra_body")
+    if extra_body is not None:
+        if not isinstance(extra_body, dict):
+            raise ValueError("Codex Responses request 'extra_body' must be an object.")
+        # Pass extra_body through verbatim — used by xAI Responses to
+        # carry `prompt_cache_key` as a body-level field (the documented
+        # cache-routing surface on /v1/responses). The openai SDK
+        # serializes extra_body into the JSON body without per-field
+        # type checks, so it survives Responses.stream() kwarg-signature
+        # changes that would otherwise raise TypeError before the wire.
+        if extra_body:
+            normalized["extra_body"] = dict(extra_body)
+
     if allow_stream:
         stream = api_kwargs.get("stream")
         if stream is not None and stream is not True: