feat(at): add backfill scripts for pieces and kidlisp
- backfill-pieces-to-atproto.mjs — syncs all unsynced pieces (186 done)
- backfill-kidlisp-to-atproto.mjs — syncs user + guest kidlisp records
supports --users-only, --guests-only, --dry-run
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>