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/tests/pty_e2e_config_ui.rs

@@ -36,8 +36,6 @@ mod reverse_agent_type_mismatch_cursor_to_default;
 mod same_agent_type_switch_no_modal;
 #[path = "pty_e2e/show_thinking_blocks_toggle_hides_existing_pty.rs"]
 mod show_thinking_blocks_toggle_hides_existing_pty;
-#[path = "pty_e2e/subscription_watch_and_gate_verify_pty.rs"]
-mod subscription_watch_and_gate_verify_pty;
 #[path = "pty_e2e/undo_tip_resets_each_new_session.rs"]
 mod undo_tip_resets_each_new_session;
 #[path = "pty_e2e/undo_tip_seen_count_never_persisted.rs"]