M1/F1: Kimi Code OAuth device-code flow

Replace the xAI OAuth stack with the Kimi device authorization grant:
- kimi_oauth.rs wire layer (device_authorization + token poll + refresh
  against kigi_env::oauth_host(); client_id per PRD; retryable statuses
  429/5xx with backoff; expired_token restarts authorization)
- X-Msh-Device-{Name,Model,Id} headers; device_id minted uuid4-hex at
  ~/.kigi/device_id (0600)
- Storage: system keyring service `kigi`, entry `oauth/kimi-code`
  (macOS/Windows native backends), atomic-file fallback under ~/.kigi;
  official client's keyring/~/.kimi never touched
- Refresh manager: 60s tick, threshold max(300, expires_in*0.5),
  401-tombstone keyed by rejected refresh token with 300s cooldown and
  rotation auto-clear, cross-process lock with sibling-adoption
  triple-check, sleep/wake forced refresh
- Deleted xAI machinery: enterprise OIDC (PKCE/JWKS/teams), devbox login,
  external auth provider, JWT tier gating + subscription paywall stack,
  X-XAI-Token-Auth marker headers, ZDR gates, /user enrichment
- kigi login / TUI /login both drive the device flow; login-host display
  now derives from kigi_env::oauth_host()
- 264 auth unit/wiremock tests; live contract probe of
  auth.kimi.com/api/oauth/device_authorization matches the wire shapes

Gates: check/clippy --all-targets clean, fmt, deny ok, kigi-shell lib
5131 tests green.

crates/codegen/kigi-shell/src/extensions/billing.rs

@@ -213,10 +213,6 @@ async fn handle_get_billing(agent: &MvpAgent) -> ExtResult {
     let credits_resp = crate::http::shared_client()
         .get(&credits_url)
         .header("Authorization", format!("Bearer {}", auth.key))
-        .header(
-            "X-XAI-Token-Auth",
-            crate::auth::GrokComConfig::default().token_header,
-        )
         .header("x-userid", &auth.user_id)
         .header("x-grok-client-version", kigi_version::VERSION)
         .header(
@@ -304,10 +300,6 @@ async fn handle_get_auto_topup_rule(agent: &MvpAgent) -> ExtResult {
     let response = crate::http::shared_client()
         .get(&url)
         .header("Authorization", format!("Bearer {}", auth.key))
-        .header(
-            "X-XAI-Token-Auth",
-            crate::auth::GrokComConfig::default().token_header,
-        )
         .header("x-userid", &auth.user_id)
         .header("x-grok-client-version", kigi_version::VERSION)
         .header(