feat: add native Antigravity (agy) integration

[uvayankee]

Closes #711

Summary

Adds native integration for the Google Antigravity (agy) coding agent:

- **Agent Identity & Integration**: Adds `SkillAgent.antigravity` with display name "Antigravity", config directory `~/.gemini/antigravity-cli`, and `antigravity-mark` asset set.
- **Hooks & Settings Installer**: Implements `AntigravityHookSettings` and `AntigravitySettingsInstaller` to register managed presence hooks in `~/.

gemini/config/hooks.jsonand install thesupacode-cliskill into~/.gemini/antigravity-cli/skills/supacode-cli/SKILL.md. - **Sidebar & UI**: Integrates Antigravity into DeveloperSettingsView, AgentAvatarGroupView, and updates CodingAgentsSidebarCardView's re-engagement cutoff date for the launch. - **Testing**: Includes full unit test coverage in AntigravitySettingsInstallerTests, SkillAgentTests, AgentPresenceOSCTests, and CodingAgentsSidebarCardModeTests`.

Type of change

• Bug fix (the linked issue is a bug report)
• Feature (the linked issue is a feature request marked ready)
• Documentation
• Other (please describe)

How was this tested?

• make check passes (format + lint)
• make test passes
• I built and ran the app to confirm the change works

AI tool disclosure (optional)

• Model(s): Gemini Flash 3.5/3.6 & Opus 4.6
• Harness / tools: Antigravity CLI

Checklist

• This pull request is linked to an issue with Closes # above.
• For a feature, the linked issue is labeled ready.
• I am the author of this work and accountable for it; no commit is authored or co-authored by an AI agent.
• I have read the Contributing guide and the Code of Conduct.