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/credential_sources.py

@@ -265,6 +265,31 @@ def _remove_minimax_oauth(provider: str, removed) -> RemovalResult:
     return result
 
 
+def _remove_xai_oauth_loopback_pkce(provider: str, removed) -> RemovalResult:
+    """xAI OAuth tokens live in auth.json providers.xai-oauth — clear them.
+
+    Without this step, ``hermes auth remove xai-oauth <N>`` silently undoes
+    itself: the central dispatcher only removes the in-memory pool entry,
+    leaves ``providers.xai-oauth`` in auth.json intact, and on the next
+    ``load_pool("xai-oauth")`` call ``_seed_from_singletons`` re-seeds the
+    entry from the still-present singleton — credentials reappear with no
+    user feedback. Clearing the singleton in step with the suppression set
+    by the central dispatcher makes the removal stick.
+
+    Belt-and-braces against the manual entry path: ``hermes auth add
+    xai-oauth`` produces a ``manual:xai_pkce`` entry whose removal step
+    falls through to "unregistered → nothing to clean up" (correct —
+    manual entries are pool-only).
+    """
+    result = RemovalResult()
+    if _clear_auth_store_provider(provider):
+        result.cleaned.append(f"Cleared {provider} OAuth tokens from auth store")
+    result.hints.append(
+        "Run `hermes model` → xAI Grok OAuth (SuperGrok Subscription) to re-authenticate if needed."
+    )
+    return result
+
+
 def _remove_codex_device_code(provider: str, removed) -> RemovalResult:
     """Codex tokens live in TWO places: our auth store AND ~/.codex/auth.json.
 
@@ -397,6 +422,11 @@ def _register_all_sources() -> None:
         remove_fn=_remove_codex_device_code,
         description="auth.json providers.openai-codex + ~/.codex/auth.json",
     ))
+    register(RemovalStep(
+        provider="xai-oauth", source_id="loopback_pkce",
+        remove_fn=_remove_xai_oauth_loopback_pkce,
+        description="auth.json providers.xai-oauth",
+    ))
     register(RemovalStep(
         provider="qwen-oauth", source_id="qwen-cli",
         remove_fn=_remove_qwen_cli,