Two new contributor rules in CLAUDE.md

The ElevenLabs voice clone is now reserved for videos on the site (the hero video, anything for obeya.si). Demo videos published elsewhere, such as in PR or issue comments, use the voice from the demo settings. A new rule also covers external pull requests: once the owner approves the demo, the demo link goes on the PR as a comment, the PR is squash-merged, and main is pulled locally.