chore: rename codex instructions (#154)

This pull request includes a minor documentation update. The
`.github/codex-instructions.md` file was renamed to `.codex/AGENTS.md`,
and the introductory section was slightly revised for clarity. No
functional or code changes were made.

Author: robodev-r2d2

.github/codex-instructions.md .codex/AGENTS.md.github/codex-instructions.md renamed to .codex/AGENTS.md

@@ -1,5 +1,4 @@
-# .github/codex-instructions.md
-
+# AGENTS.md
 > **Scope:** These instructions tune our **Codex** code-generation agent (LLM via API/CLI) for this monorepo. They are **authoritative**: follow them over generic best practices. Output must compile, pass linters/tests, and fit our structure.
 
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