feat: add Google Antigravity CLI (agy) as a supported agent

[rdh073]

Summary

• Add Google Antigravity CLI (agy) to the dmux agent registry.
• Detect agy from PATH and common install locations.
• Launch initial prompts with agy --prompt-interactive so the session remains interactive.
• Add Antigravity pane attention heuristics coverage, registry tests, and docs references.

Antigravity CLI facts

• Binary: agy.
• Install command: curl -fsSL https://antigravity.google/cli/install.sh | bash.
• Config directory: ~/.gemini/antigravity-cli/.
• Project directory: .antigravitycli/.
• Resume flag: --continue / -c.
• Permission bypass flag: --dangerously-skip-permissions.
• Headless mode exists via --print, -p, and --prompt, but print mode has had non-TTY stdout issues.

Prompt transport choice

This uses promptTransport: 'option' with promptOption: '--prompt-interactive'.

Local agy --help on agy 1.0.3 describes --prompt-interactive as running an initial prompt interactively and continuing the session, which matches dmux's pane workflow better than pasting with tmux send-keys. It also avoids relying on headless print mode.

Antigravity is registered as defaultEnabled: false, matching the optional-agent pattern for newly supported agents.

Test plan

• pnpm install
• pnpm exec vitest --run __tests__/agentLaunch.test.ts
• pnpm exec tsc --noEmit
• pnpm --filter dmux-docs run build