feat: add Antigravity IDE support with dedicated rules path

[biisal]

closes #116

This PR adds support for Antigravity IDE by introducing the workspace-level rules file under the expected directory structure .agents/rules/.

To prevent future rule drifts when the master AGENTS.md file is updated, this PR also updates the check-rule-copies.js script to validate and assert that the Antigravity rules file stays perfectly in sync with the rest of the rule copies.

Changes Made

• [NEW] Added .agents/rules/ponytail.md: Configured the compact always-on ponytail rule set for the workspace under Antigravity IDE.
• [MODIFY] Updated scripts/check-rule-copies.js: Appended .agents/rules/ponytail.md to the rule copy verification array to automatically prevent future code/instruction drift.

Verification

Ran the rule check script locally to confirm that the new copy perfectly matches the canonical AGENTS.md:

node scripts/check-rule-copies.js

[DietrichGebert]

Thanks! #119 is the same Antigravity rule copy and it's already green on CI, so I'm taking that one. Closing this as a duplicate of #119, no reflection on the work, they just landed the same change.