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-tui/src/app/dispatch/auth.rs

@@ -45,15 +45,9 @@ pub(super) fn ensure_login_method(app: &mut AppView) {
     // No interactive method: leave login_method_id unset (fail-closed).
 }
 
-/// Error when no interactive login method is available (empty auth_methods,
-/// e.g. `preferred_method=api_key` with no credentials). Prefer the shell's
-/// pin-unavailable copy when the list is empty.
-fn no_login_method_error(app: &AppView) -> String {
-    if app.auth_methods.is_empty() {
-        kigi_shell::agent::auth_method::PREFERRED_API_KEY_UNAVAILABLE.to_string()
-    } else {
-        "No login method available".to_string()
-    }
+/// Error when no interactive login method is available (empty auth_methods).
+fn no_login_method_error(_app: &AppView) -> String {
+    "No login method available".to_string()
 }
 
 /// Log out, then start a new login flow in a single sequential task.