Add antigravity support and generic overlays structure

[jed2nd]

Summary

Add support for antigravity

Linked item

• Implements: Add support for Google Antigravity #305 (must be a Discussion)

Checklist

• Linked to related Issue/Discussion
• Documented steps to test (below)
• Drafted “how to use” docs (if this adds new behavior)
• Backwards compatibility considered (notes if applicable)

Documented steps to test

Verify Antigravity install:

1. Create a test folder: mkdir test && cd test
2. Run install: ~/agent-os/scripts/project-install.sh --antigravity
3. Verify destination: Check that .agent/workflows/ exists.
4. Verify content: Check that cat .agent/workflows/shape-spec.md contains "PLANNING mode".
5. Verify frontmatter: Check that head .agent/workflows/discover-standards.md shows the YAML description block.

Verify Claude Code install:

1. Create a test folder: mkdir test && cd test
2. Run install: ~/agent-os/scripts/project-install.sh
3. Output says "Installing commands... to .claude/commands/agent-os/"
4. Check file location: ls .claude/commands/agent-os/ should list command files.
5. Check content: cat .claude/commands/agent-os/shape-spec.md should contain "plan mode" (lowercase, original version).
6. Check frontmatter: Files should NOT have automatically injected description frontmatter (unless present in source).

Notes for reviewers

Generic overlay structure allows non-claude-code based installations, since there is so much direct reference to claude or claude specific structures in the existing markdown