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.
Lakshya77089