docs: add new CLAUDE.md file and reference AGENTS.md
Description
Adds a new CLAUDE.md file that points to AGENTS.md.
For reference, see Agents.md.
This is a follow-up to the discussion on !3450 (closed). Rather than maintaining a
separate CLAUDE.md file with duplicated content (which risks drifting out of
sync), CLAUDE.md references AGENTS.md.
Edited by Brendan Lynch