Unify agent instructions to AGENTS.md

[Sha1rholder]

1. Both Antigravity and Claude Code now natively support AGENTS.md. These and some other "curious" agents sometimes explore these symlinks so repeatedly read .rules, wasting attention.
2. Zed Agent now supports AGENTS.md first-class. So no longer need .rules. I suggest place its contents in AGENTS.md and remove it.

Basically a follow-up of #45939

[Sha1rholder]

Drafted a PR for this: #66716. Not sure if people like it.