feat(agy): enhance Google Antigravity CLI integration (#2689)

* feat(agy): enhance Google Antigravity CLI integration

- Set requires_cli=True and install_url for CLI tool detection
- Implement build_exec_args() for non-interactive execution via agy --print
- Add dot-to-hyphen hook command note injection in generated SKILL.md files

* fix(agy): add --ignore-agent-tools to TestAgyAutoPromote tests

Tests verify file layout and setup warnings, not CLI presence.
agy requires_cli=True causes CI failures when agy is not installed.