docs: Add AI notes with conventional commits guidance (#2891)

## Summary
- Adds `.ai/index.md` with a note to use conventional commits for PR
titles.

## Test plan
- [x] N/A — documentation only

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Saadnajmi

.ai/index.md

@@ -0,0 +1,3 @@
+# AI Notes
+
+Use [Conventional Commits](https://www.conventionalcommits.org/) for PR titles.