ci: genuinely rebuild caveman.skill, ship pi-v* tags, verify every plugin mirror

zip -r adds to an existing archive and dist/caveman.skill is tracked, so a file
deleted from skills/caveman/ never left the shipped ZIP. Delete before zipping.
release-packages also triggers on pi-v*, and verify_repo checks all six mirrors
sync-skill.yml writes instead of only the first.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SqmbqWDtNexzDfH5V3BMRz