Add Antigravity IDE agent ruleset configuration

[biisal]

Description

Antigravity IDE supports loading rules from .agents/rules/ directory structure for repo/workspace rules. To make the ponytail rule set fully portable and always-on for developers using Antigravity IDE, we should add a dedicated config copy at .agents/rules/ponytail.md.

Proposed Changes

1. Add Rules File: Create .agents/rules/ponytail.md containing the ponytail rules set.
2. Prevent Drift: Add the new file to the check-rule-copies.js script so it validates successfully against the canonical AGENTS.md and doesn't drift during future updates.

[Lakshya77089]

Opened a PR for this: #119. Adds .agents/rules/ponytail.md as a verbatim copy of the canonical AGENTS.md body and registers it in check-rule-copies.js so it's drift-checked — exactly the two changes proposed here. Drift guard passes and
pm test is green (51/51).

[biisal]

It seems like you did the same pr I have already created #117