CLAUDE.md: new rule on voices in demo videos

A new bullet, placed between the rule on checking demo videos before handover and the rule on the site's narration voice.

- Before a demo video goes to the owner, watch it for faults of the demo itself and fix them first:
  …
- When a demo video shows a feature that speaks itself (the narrated walkthrough, say), that
  feature speaks in a different voice than the video's narration (Kokoro for the walkthrough), so
  viewers can tell the demo's narrator from the presentation inside it.
- Videos on the site (the hero video, anything for obeya.si) are narrated in the owner's ElevenLabs
  …