feat(providers): add xAI Grok subscription OAuth (device-code) + per-provider session auth

First subscription-OAuth provider beyond Kimi Code (26th registry variant).
Log in with a Grok/SuperGrok/X subscription via RFC-8628 device-code OAuth
(auth.x.ai), then use it against api.x.ai/v1 — reusing the existing xai wire
(ChatCompletions + OpenAI listing + Passthrough + restrict + models_dev_id
xai). Sourced from Pi (earendil-works/pi auth/oauth/xai.ts): client
b1a00492..., scope 'openid profile email offline_access grok-cli:access
api:access', standard Bearer (no x-xai-token-auth).

Foundation (generalizes Kigi's Kimi-singleton OAuth to per-provider, root
cause, not a patch):
- Registry: OAuthConfig on PlatformSpec (client_id/host/device+token
  paths/scope/scope_key); XAI_OAUTH_CONFIG + XAI_GROK_SPEC (uses_oauth, method
  id 'xai-grok', an interactive login after kimi-code).
- Generic device-code wire (auth/oauth_device.rs) + GenericDeviceRefresher,
  sharing the RFC-8628 core with Kimi; Kimi's bespoke flow is byte-identical
  (X-Msh headers, KIMI_CODE_OAUTH_SCOPE, keyring gating unchanged).
- Per-provider AuthManager via a process-global pool (auth/oauth_registry.rs):
  build-on-demand with start_proactive_refresh, keyed by scope. The session
  resolves the AuthManager for the ACTIVE model's platform for bearer/refresh/
  401-recovery/api_key — an oauth-platform model always uses its OWN token,
  never the primary.
- Live /models under OAuth; base routes oauth().is_some() -> platform.base_url()
  (kimi-code stays on proxy_url).

Security: adversarial review + a systematic token-leak audit found and closed
FIVE channels where the primary Kimi token could reach api.x.ai (bearer
resolver, api_key stamping, aux summary/classifier/image-describe models, and
subagent model-override). Each fix routes through the platform-aware resolver
(the oauth model's pooled token or None, NEVER the primary) and is revert-to-red
verified. No access/refresh token is ever logged.

Registry at 26; picker updated (xai-grok interactive login row); TUI
context-window already auto-updates per model. Full gate green (234 suites,
fmt, clippy -D warnings, deny). GPT/Claude/Grok officially permit third-party
subscription use.

crates/codegen/kigi-shell/src/agent/subagent/mod.rs

@@ -982,9 +982,20 @@ fn resolve_model_override_to_config(
     } else {
         acp::ModelId::new(entry.info().model.clone())
     };
-    let session_key = ctx.auth.as_ref().map(|a| a.key.as_str());
+    // Resolve the child's session token by the OVERRIDE model's OWN platform,
+    // not the parent's primary auth: a grok (oauth-platform) override draws its
+    // pooled grok token or `None` — NEVER the primary Kimi session token (which
+    // `resolve_credentials` would otherwise stamp onto the child's api.x.ai
+    // credentials, leaking it in the logout-mid-session edge). A first-party /
+    // non-oauth override still resolves to the primary (byte-identical).
+    let managed_key = entry.info().id.as_deref().unwrap_or(model_id);
+    let session_key = crate::auth::oauth_registry::session_key_for_model(
+        &crate::util::kigi_home::kigi_home(),
+        managed_key,
+        Some(&ctx.auth_manager),
+    );
     let has_session_key = session_key.is_some();
-    let mut credentials = resolve_credentials(&entry, session_key);
+    let mut credentials = resolve_credentials(&entry, session_key.as_deref());
     credentials.auth_type = subagent_auth_type(Some(&entry), &ctx.auth_method_id);
     let resolved_auth_type = credentials.auth_type;
     let config = sampling_config_for_model(&entry, credentials, ctx.alpha_test_key.clone());