feat: add Antigravity .agents/rules/ponytail.md rule copy (#119)

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.

scripts/check-rule-copies.js

@@ -20,6 +20,7 @@ const copies = [
   ['.cursor/rules/ponytail.mdc', stripFrontmatter],
   ['.windsurf/rules/ponytail.md', text => text.trim()],
   ['.clinerules/ponytail.md', text => text.trim()],
+  ['.agents/rules/ponytail.md', text => text.trim()],
   ['.github/copilot-instructions.md', text => text.trim()],
   ['.kiro/steering/ponytail.md', stripFrontmatter],
 ];