Close linked sub-issues automatically on PR merge (squash-safe) #368

GitHub's "Closes #N" keyword is only evaluated against the final commit
message that lands on main. On a squash-merge repo, that's the squash
commit box GitHub pre-fills from the commit(s) in the PR — not the PR
description — so a "Closes #N" trailer in the PR body alone doesn't
reliably close anything.

Case in point: PR #367 (Issue #362) merged with "Closes #362" in the PR
body, but the squash commit title only had "(#362)" (a bare reference,
not a closing keyword). Issue #362 never closed; confirmed via GraphQL
that closingIssuesReferences was empty and willCloseTarget was false.

Add a GitHub Action on pull_request (types: [closed], if merged == true)
that parses Issue- out of head.ref (same pattern scripts/flow.py's
BRANCH_RE already uses) and closes that sub-issue directly via the API,
plus the parent issue when it was the last open sibling group (same
sibling-check logic already in scripts/flow.py's cmd_pr). This removes
the dependency on GitHub's commit-message keyword parsing entirely, in
line with the other enforced-not-documented mechanisms in CLAUDE.md.