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/mux-interface.ts

@@ -17,6 +17,7 @@ import type {
   CodexConfig,
   EffortLevel,
   GeminiConfig,
+  AntigravityConfig,
   SessionRemote,
   SessionDocker,
 } from './types.js';
@@ -74,6 +75,7 @@ export interface CreateSessionOptions {
   openCodeConfig?: OpenCodeConfig;
   codexConfig?: CodexConfig;
   geminiConfig?: GeminiConfig;
+  antigravityConfig?: AntigravityConfig;
   /** When restoring after reboot, resume a previous Claude conversation by its session ID */
   resumeSessionId?: string;
   /** Extra env vars exported before launching the CLI (e.g., CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS). Ephemeral — not written to disk. */
@@ -102,6 +104,7 @@ export interface RespawnPaneOptions {
   openCodeConfig?: OpenCodeConfig;
   codexConfig?: CodexConfig;
   geminiConfig?: GeminiConfig;
+  antigravityConfig?: AntigravityConfig;
   /** Resume a previous Claude conversation when respawning */
   resumeSessionId?: string;
   /** Extra env vars exported before launching the CLI (preserved across respawns). */