Keep doc-reference checks aligned with AGENTS.md instruction content

`scripts/check_doc_refs.py` lists only `CLAUDE.md` in its default documentation globs. If the repository instructions ever move to `AGENTS.md` while `CLAUDE.md` becomes a pointer, the check scans only the pointer and silently stops validating the actual instructions.

Update the default globs to include `AGENTS.md` whenever that repository convention is adopted. The check must continue scanning the real instruction content, so a passing result cannot cover only a pointer file.

Start in scripts/check_doc_refs.py and inspect its default documentation globs and how it handles CLAUDE.md pointers. Ensure the defaults include AGENTS.md when that repository convention is adopted, so the check reads the real instruction content rather than only the pointer. Run the document-reference check and confirm a passing result covers AGENTS.md content.

Written by the indexing model from the issue text.