chore: persist Antigravity AI co-author configuration for the project

- Added Antigravity commit co-authoring rules to CLAUDE.md
- Created .agent/rules.md baseline to explicitly bind Antigravity to appending its co-author attribution

Co-authored-by: Antigravity <antigravity@deepmind.com>

CLAUDE.md

@@ -25,3 +25,8 @@ Once a tech stack is chosen and implementation begins, update this file with:
 - Build, test, and lint commands
 - Architecture overview
 - Development setup instructions
+
+## Git Commits
+
+When making git commits, always append the following co-author trailer to the end of the commit message to indicate AI assistance:
+`Co-authored-by: Antigravity <antigravity@deepmind.com>`