chore: add a CLAUDE.md bridge to AGENTS.md

[maehr]

Problem

This repository holds AGENTS.md and no CLAUDE.md. Claude Code reads CLAUDE.md and ignores AGENTS.md. An agent that clones this repository directly therefore loads no instructions.

git ls-files | grep -iE 'claude|agents'   # prints AGENTS.md only

The gap appears when a contributor works in textrefs/registry on its own. A contributor who works through the data/ submodule of textrefs/textrefs.org is not affected, because the parent repository loads its own bridge file.

Proposed change

1. Add CLAUDE.md at the repository root.
2. Give the file the single line @AGENTS.md.
3. Track the file with git.

Do not copy any content from AGENTS.md into CLAUDE.md. AGENTS.md stays the one machine contract, and CLAUDE.md stays the bridge.

Acceptance criteria

• git ls-files CLAUDE.md prints CLAUDE.md.
• CLAUDE.md contains @AGENTS.md and nothing else.

Context

This issue is item A2 of textrefs/textrefs.org#125. The matching change for the site repository is textrefs/textrefs.org#141.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NtQMT2WCNxEAEbEbdhVmuX