handover-flush

When the user asks to flush handover branches, run:

bash scripts/handover/flush.sh [--dry-run] [--cleanup] [--no-pr-open]

Walks every local handover/* branch in the resolved handover repo and
reconciles each against origin (push unpushed, open missing PRs, report merged;
--cleanup deletes branches that landed on main). Best-effort — exits 0 even on
per-branch errors. Report the per-branch status table. See
.claude/commands/handover-flush.md for detail.