feat(agents): add Antigravity CLI (agy) as built-in agent

[ASRagab]

Add Antigravity CLI (agy) support

Fixes #152

Google is migrating Gemini CLI to the new Antigravity CLI (agy). On June 18, 2026 Gemini CLI stops serving Google AI Pro/Ultra and free Code Assist users; it remains only on paid/enterprise API keys. This adds Antigravity as a first-class agent so users have a working migration path — without removing Gemini CLI, which paid/enterprise users still rely on. Fully additive, no breaking change.

What changed

• Agent registry — new antigravity entry in DEFAULT_AGENTS (electron/ipc/agents.ts): command: agy, resume -c, skip-permissions --dangerously-skip-permissions, prompt_ready_delay_ms: 1000. Launched interactively like every other agent (prompt typed into the TUI, not passed as -p).
• Docker (forward-compatible scaffolding only) — ~/.gemini/antigravity-cli added to the shared-auth mount map (AGENT_CONFIG_DIRS) for settings/plugins; agy installed in the bundled image via the official installer (Go binary, not npm). See the auth note below — Antigravity is native-only.
• MCP guard — agy excluded from both --mcp-config emission paths (legacyMcpConfigArgs in src/lib/agent-args.ts and buildMcpLaunchArgs in electron/mcp/agent-args.ts), the same way Codex is excluded. agy has no --mcp-config flag; passing it would break launch. Scope consequence (intentional): because buildMcpLaunchArgs returns nothing for agy, Antigravity tasks get no parallel-code MCP coordinator wiring — i.e. no spawning of parallel-code subagents from inside an agy session. Safe default for this change; wiring agy into the coordinator (it configures MCP via plugins/config, not a CLI flag) is a follow-up if wanted.
• Gemini CLI untouched.

⚠️ Antigravity is native-only (no Docker isolation)

• Native (non-Docker) tasks work fully via the normal interactive sign-in — credentials cached in the host OS keyring (Keychain/libsecret).
• Docker-isolated Antigravity cannot authenticate. Login is keyring-only OAuth; a barebones agent container has no secret-service daemon to reach the keyring, and agy has no API-key environment fallback (an earlier ANTIGRAVITY_API_KEY assumption was unverified and is false — agy still enters OAuth with it set). Run Antigravity as a native task.
• The Docker bits are kept as additive scaffolding: the bundled image ships the agy binary and ~/.gemini/antigravity-cli (settings/plugins) is mounted when "Share agent auth" is enabled, so a future file-based / API-key auth path drops in cleanly.

Corrections from Codex review

• Mount path fixed: agy writes app data to ~/.gemini/antigravity-cli, not ~/.config/antigravity (verified against the installed binary).
• Removed the unsupported ANTIGRAVITY_API_KEY Docker auth claim (binary does not consume it).
• data/ runtime state (memory/sidecar state_store.db / stream_store) gitignored — it is generated local state, not source.

Verification

• tsc --noEmit clean; full test suite green (111 tests across persistence, agents, both agent-args, pty). pty.test.ts config-dir-mount table extended with the corrected agy path.
• New unit tests assert no --mcp-config is emitted for agy on both arg paths.
• Manual native smoke test (selector availability, prompt delivery, -c resume, skip-permissions) and a full docker build are pending hardware/Docker.

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