handover-pr-open

When the user asks to open or update the handover PR, run:

bash scripts/handover/pr-open.sh [--dry-run] [--base <branch>]

Opens a new PR or updates the existing one for the current handover branch
(idempotent). Refuses (rc=3) if HEAD is not on a handover/* branch. Run from
the worktree of the handover repo (not himmel root). See
.claude/commands/handover-pr-open.md for env vars (HANDOVER_PR_AUTO,
HANDOVER_PR_BASE, GH_CMD) + exit codes.