feat(cli): Antigravity run mode

Adds Antigravity as a sixth CLI backend alongside Claude Code, shell, OpenCode,
Codex and Gemini, following the existing pluggable-resolver pattern.

- `utils/antigravity-cli-resolver.ts` resolves the CLI, mirroring the other
  resolvers; `GET /api/antigravity/status` reports availability and path.
- `ANTIGRAVITY_*` joins the `ALLOWED_ENV_PREFIXES` allowlist in schemas.ts, so
  env overrides stay CLI-scoped rather than blanket-forwarded.
- Session, tmux-manager, mux-interface and types carry the new mode; secrets are
  injected via socket-scoped `tmux setenv`, never on the spawn command line, so
  the mode requires tmux with no direct PTY fallback like the other external CLIs.
- Frontend: Run-dropdown entry, agent-type option, `ag` tab badge and toolbar
  colours. `runAntigravity()` routes remote/docker cases through
  `POST /api/quick-start` and skips the local status probe for them.

Tests: test/antigravity-mode.test.ts, plus run-mode-ui and system-routes coverage.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/types/session.ts

@@ -8,12 +8,13 @@
  * - SessionConfig — creation-time config (id, workingDir, createdAt)
  * - SessionOutput — captured stdout/stderr/exitCode
  * - SessionStatus — 'idle' | 'busy' | 'stopped' | 'error'
- * - SessionMode — 'claude' | 'shell' | 'opencode' | 'codex' | 'gemini' (which CLI backend)
+ * - SessionMode — 'claude' | 'shell' | 'opencode' | 'codex' | 'gemini' | 'antigravity' (which CLI backend)
  * - ClaudeMode — CLI permission mode ('dangerously-skip-permissions' | 'auto' | 'normal' | 'allowedTools')
  * - SessionColor — visual differentiation color
  * - OpenCodeConfig — OpenCode-specific settings (model, autoAllowTools, continueSession)
  * - CodexConfig — Codex (OpenAI CLI)-specific settings (model, resumeSessionId)
  * - GeminiConfig — Gemini CLI-specific settings (model, approvalMode, resumeSession)
+ * - AntigravityConfig — Antigravity CLI (agy) settings (model, dangerouslySkipPermissions, resumeConversationId)
  *
  * Cross-domain relationships:
  * - SessionState.respawnConfig embeds RespawnConfig (respawn domain)
@@ -42,9 +43,12 @@ export type SessionStatus = 'idle' | 'busy' | 'stopped' | 'error';
 export type ClaudeMode = 'dangerously-skip-permissions' | 'auto' | 'normal' | 'allowedTools';
 
 /** Session mode: which CLI backend a session runs */
-export type SessionMode = 'claude' | 'shell' | 'opencode' | 'codex' | 'gemini';
+export type SessionMode = 'claude' | 'shell' | 'opencode' | 'codex' | 'gemini' | 'antigravity';
 
-export type RemoteCommandMode = Extract<SessionMode, 'shell' | 'claude' | 'opencode' | 'codex' | 'gemini'>;
+export type RemoteCommandMode = Extract<
+  SessionMode,
+  'shell' | 'claude' | 'opencode' | 'codex' | 'gemini' | 'antigravity'
+>;
 
 /**
  * Advanced SSH connection options shared by RemoteHost and SessionRemote.
@@ -150,7 +154,10 @@ export interface RemoteSessionInfo {
 // into the same long-lived container. See `docs/docker-cases-plan.md`.
 
 /** Which CLI backends a Docker case can run (same set as remote). */
-export type DockerCommandMode = Extract<SessionMode, 'shell' | 'claude' | 'opencode' | 'codex' | 'gemini'>;
+export type DockerCommandMode = Extract<
+  SessionMode,
+  'shell' | 'claude' | 'opencode' | 'codex' | 'gemini' | 'antigravity'
+>;
 
 /** Container engine. Docker and Podman differ in the uid/userns + host-gateway alias. */
 export type DockerEngine = 'docker' | 'podman';
@@ -312,6 +319,16 @@ export interface GeminiConfig {
   resumeSession?: string;
 }
 
+/** Antigravity CLI (agy) session configuration */
+export interface AntigravityConfig {
+  /** Model identifier. Passed via --model. */
+  model?: string;
+  /** Auto-approve all tool permission requests (passes --dangerously-skip-permissions). Absent = agy's default prompting. */
+  dangerouslySkipPermissions?: boolean;
+  /** Resume a previous conversation by ID (passed via --conversation). */
+  resumeConversationId?: string;
+}
+
 /**
  * Configuration for creating a new session
  */
@@ -453,6 +470,8 @@ export interface SessionState {
   codexConfig?: CodexConfig;
   /** Gemini-specific configuration (only for mode === 'gemini') */
   geminiConfig?: GeminiConfig;
+  /** Antigravity-specific configuration (only for mode === 'antigravity') */
+  antigravityConfig?: AntigravityConfig;
   /** Claude conversation session ID to resume after reboot (set by restore script) */
   resumeSessionId?: string;
   /** Claude CLI effort level (soft default via --settings, switchable in-session via /effort) */