Antigravity IDE loads always-on rules from .agents/rules/. The README already tells Antigravity users to drop the ruleset there, but the file was missing. Add it as a verbatim copy of the canonical AGENTS.md body (no frontmatter, like the .windsurf/.clinerules copies) and register it in check-rule-copies.js so it is validated against AGENTS.md and cannot drift. Closes #116.