Ops: Project board backlog sync and issue-linked commit enforcement

## Summary
Backfill project board #3 with retrospective Done cards and planned Todo work. Add Sync-ProjectBoard.ps1, commit-msg hook, PR CI check, and workflow docs.

## Acceptance criteria
- [x] Board lists all shipped work (Done) and planned work (Todo)
- [x] Sync script is idempotent
- [x] commit-msg hook + install script
- [x] PR workflow requires issue link
- [ ] Merged to main via PR