New rule in CLAUDE.md

Added as the last bullet, after the rule on contributors' pull requests.

- Once the owner approves the demo of an external contributor's pull request: comment on the PR
  with the demo link, squash-merge it, then `git pull` on `main` so the local checkout is current.
+ - Pull requests Obeya opens in other people's repositories carry no Obeya attribution: no
+   `Co-authored-by` Obeya, no mention of Obeya in the PR text. They may link the demo page, and the
+   page itself may say "created with Obeya" at the bottom.