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/transports/codex.py

@@ -89,18 +89,25 @@ class ResponsesApiTransport(ProviderTransport):
         _effort_clamp = {"minimal": "low"}
         reasoning_effort = _effort_clamp.get(reasoning_effort, reasoning_effort)
 
+        response_tools = _responses_tools(tools)
         kwargs = {
             "model": model,
             "instructions": instructions,
             "input": _chat_messages_to_responses_input(payload_messages),
-            "tools": _responses_tools(tools),
-            "tool_choice": "auto",
-            "parallel_tool_calls": True,
+            "tools": response_tools,
             "store": False,
         }
+        if response_tools:
+            kwargs["tool_choice"] = "auto"
+            kwargs["parallel_tool_calls"] = True
 
         session_id = params.get("session_id")
-        if not is_github_responses and session_id:
+        # xAI's Responses API uses `prompt_cache_key` (body-level) as the
+        # cache-routing key, not a top-level kwarg — the body-field
+        # injection below survives openai SDK builds whose
+        # Responses.stream() signature drops the kwarg. Everything else
+        # that ISN'T github/xAI keeps using the typed kwarg.
+        if not is_github_responses and not is_xai_responses and session_id:
             kwargs["prompt_cache_key"] = session_id
 
         if reasoning_enabled and is_xai_responses:
@@ -165,6 +172,22 @@ class ResponsesApiTransport(ProviderTransport):
             merged_extra_headers["x-grok-conv-id"] = session_id
             kwargs["extra_headers"] = merged_extra_headers
 
+            # xAI Responses cache-routing field. Lives in the request body
+            # (per https://docs.x.ai/.../prompt-caching/maximizing-cache-hits),
+            # so we ship it via extra_body — the openai SDK serializes
+            # extra_body fields into the JSON body without per-field type
+            # validation, sidestepping the TypeError that fires on
+            # Responses.stream() builds whose `prompt_cache_key` kwarg has
+            # been dropped. Setdefault preserves a caller-supplied value
+            # (e.g. request_overrides.extra_body.prompt_cache_key) over
+            # the auto-derived session_id.
+            existing_extra_body = kwargs.get("extra_body")
+            merged_extra_body: Dict[str, Any] = {}
+            if isinstance(existing_extra_body, dict):
+                merged_extra_body.update(existing_extra_body)
+            merged_extra_body.setdefault("prompt_cache_key", session_id)
+            kwargs["extra_body"] = merged_extra_body
+
         return kwargs
 
     def normalize_response(self, response: Any, **kwargs) -> NormalizedResponse: