chore: replace AGENTS.md with symlink to CLAUDE.md

Avoid duplicating content — Codex reads AGENTS.md which now points
directly to CLAUDE.md where all guidance (including the pro-upgrade
skill reference) lives.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AGENTS.md

@@ -1,5 +0,0 @@
-# AGENTS.md
-
-This file provides guidance for AI coding agents when working with code in this repository.
-
-For full project guidance, see [CLAUDE.md](./CLAUDE.md).

AGENTS.md

@@ -0,0 +1 @@
+CLAUDE.md