[GFI] Add Antigravity (Google) adapter

Goal

Add an antigravity IDE adapter so Coco artifacts install into Google's Antigravity editor. The README's System Compatibility table lists Antigravity as Planned for v0.2 — this issue tracks building it.

Files involved

• adapters/antigravity/manifest.json (new) — mirror an existing adapter's manifest
• adapters/antigravity/install.sh (new) — link/compile skills, commands, agents, rules into Antigravity's config location
• adapters/antigravity/README.md (new)
• install.sh — detect_adapter() may add an Antigravity heuristic

Reference

adapters/cursor/ and adapters/vscode-continue/ are the closest patterns. The generic adapter (AGENTS.md producer) is a good fallback model if Antigravity supports AGENTS.md.

Acceptance criteria

• bash install.sh --adapter antigravity installs without error
• Skills, commands, agents, and rules are wired into the correct Antigravity location
• manifest.json validates; README documents the target paths
• CI passes (frontmatter linter, manifest validator, install script syntax check)
• README System Compatibility row updated from "Planned" to "Beta/Stable"

Hints

• Adding an adapter: CONTRIBUTING.md
• Frontmatter spec: docs/architecture.md

Start with adapters/cursor/, adapters/vscode-continue/, and the generic adapter, then read CONTRIBUTING.md and docs/architecture.md. Add adapters/antigravity/manifest.json, install.sh, and README.md, update the root install.sh detection and README compatibility row, and run the documented CI checks plus bash install.sh --adapter antigravity. Done means the manifest validates, installation succeeds, all artifact types are wired to documented Antigravity paths, and CI passes.