docs: rewrite AGENTS.md and convert CLAUDE.md from symlink to standalone file

AGENTS.md was verbose and duplicated README content. Rewrote to follow the
GravityKit AGENTS.md convention with Quick Start, Repository Map, Architecture,
Conventions, Extension Patterns, and Hook Reference sections.

- CLAUDE.md was a symlink to AGENTS.md; replaced with a standalone file that
  references AGENTS.md and adds key commands, env setup, and critical rules

Author: zackkatz