feat(installer): add Antigravity IDE support as a native plugin

[miguelarcjr]

What this PR does

This PR introduces full, native support for Antigravity IDE as an installer target in the Codegraph CLI.

When users run codegraph install (or during the interactive setup), they can now select Antigravity IDE alongside Claude Code and Cursor.

Technical Details

• Plugin Architecture: Instead of modifying scattered config files, the installer packages CodeGraph as a formal Antigravity Plugin. This cleanly bundles the mcp_config.json server definitions together with the prompt instructions.
• Local & Global Support: Fully supports both --location=global (installs to ~/.gemini/config/plugins/codegraph) and --location=local (installs to the workspace .agents/plugins/codegraph/).
• Skills & Rules Integration:
  • CodeGraph's INSTRUCTIONS_TEMPLATE is bundled as an Antigravity Skill (with YAML frontmatter) allowing the agent to discover it via progressive disclosure.
  • It is simultaneously injected as an Always-On Rule (trigger: always_on) to ensure the agent reliably defaults to codegraph tools over native search.
• Cleanup: Uninstallation recursively removes the plugin directory, leaving no stale configuration behind.

How to test

1. Build the CLI locally (npm run build).
2. Run node dist/bin/codegraph.js install --target=antigravity --location=local --yes.
3. Verify that the .agents/plugins/codegraph directory is created with plugin.json, mcp_config.json, skills/, and rules/.
4. Run node dist/bin/codegraph.js uninstall --target=antigravity --location=local --yes to verify cleanup.