feat(antigravity): implement pristine, zero-redundancy OAC-to-Antigravity bridge

[benpriebe]

Description

This PR establishes a clean, portable, and backwards-compatible integration bridge between OpenAgents Control (OAC) and Google Antigravity (gemini / agy).

It implements a Node-based workspace setup utility that dynamically generates wrapped runtime files inside the gitignored .agents/ folder and packages a standard workspace plugin (openagents-control-bridge).

By using in-memory frontmatter wrapping and a gitignored .agents/ runtime directory, this integration guarantees:

1. 100% Pristine .opencode/: The original .opencode/ directory is completely untouched and unmodified. There is absolutely zero risk of breaking native OpenCode execution!
2. Zero Repository Redundancy: No duplicates are tracked or committed to Git. The original .opencode/ folder remains the single source of truth.
3. IDE Scannability: Root-level files under .agents/ allow the Antigravity chat UI to index slash-commands and custom subagents in autocomplete, while the plugin directory packages them.
4. Standalone Global Plugins: The installer uses dereferenced copying (cp -RL) to compile a completely standalone, portable plugin globally under ~/.gemini/ that works out-of-the-box across all other projects.

Type of Change

• New feature (agent, command, tool)
• Bug fix
• Documentation
• Refactoring
• CI/CD

Checklist

• Tests pass locally
• Documentation updated (if needed)
• Follows CONTRIBUTING.md

Testing

• Validated setup locally by running ./integrations/antigravity/install-antigravity.sh and node scripts/setup-gemini-bridge.js.
• Confirmed that all original .opencode/ files are completely untouched and unmodified (git diff is completely empty for .opencode/).
• Verified that all generated local agent and skill files under .agents/ have fully formed Antigravity frontmatter (name:, model:, tools:).

---

Note: Registry validation and smoke tests will run automatically.