docs: simplify CLAUDE shim (#84)

## Summary
- normalize `CLAUDE.md` to the single-line `@AGENTS.md` shim

## Test Plan
- `git diff --check`

---------

Co-authored-by: Codex <noreply@openai.com>

Author: jbeckwith-oai

CLAUDE.md

@@ -1,5 +1 @@
-# Compatibility Note
-
-The canonical repository guide is [AGENTS.md](./AGENTS.md).
-
-This file remains as a compatibility shim for tools that still auto-load `CLAUDE.md`.
+@AGENTS.md