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/utils/index.ts

@@ -30,3 +30,4 @@ export { findClaudeDir, getAugmentedPath, getClaudeCliVersion } from './claude-c
 export { resolveOpenCodeDir } from './opencode-cli-resolver.js';
 export { resolveCodexDir, isCodexAvailable } from './codex-cli-resolver.js';
 export { resolveGeminiDir, isGeminiAvailable } from './gemini-cli-resolver.js';
+export { resolveAntigravityDir, isAntigravityAvailable } from './antigravity-cli-resolver.js';