chore: add safety notes to release command

This PR adds two safety notes to the Claude Code release command:
- Mathlib bump branches live on `mathlib4-nightly-testing`, not the main `mathlib4` repo
- Never force-update remote refs without explicit user confirmation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
1 file changed