handover-pr-merge

When the user asks to merge the handover PR, run:

bash scripts/handover/pr-merge.sh [--dry-run]

Fires a plain gh pr merge <N> --squash --delete-branch for the open PR on the
current handover branch — no --admin (that fallback needs explicit
GH_ADMIN_MERGE_OK=1; the script never silently escalates). Refuses (rc=3) if
HEAD is not on a handover/* branch. Run from the worktree of the handover repo.
Run after handover-flush or at session-end. See
.claude/commands/handover-pr-merge.md for env vars + exit codes.