feat(hooks): live-sync keeps live checkouts on their upstream; post-merge-cleanup leaves server-owned deletes alone

live-sync (SessionStart). Config lives in a repo and ~/.claude links into it; if
the links point at a working clone, what runs is whatever branch that clone is on.
A merge tool ran from a feature branch 165 commits behind main for a month. The
fix is a checkout nobody edits plus this hook: ~/.claude/live-checkouts lists the
copies; each is fast-forwarded (branch: merge --ff-only, detached: checkout
--detach) and then runs its post-sync command. Tracked edits, own commits, fetch
or command failures are reported and the copy is left alone. git refuses to walk
a branch off its own commits but not a detached HEAD, so the check is ours; the
mutation without it goes red. 21 cases; 8 guards mutated, 7 red, the 8th is an
equivalent mutant (merge --ff-only on a detached HEAD does the same).

post-merge-cleanup: when the MR asks the server to delete its branch
(force_remove_source_branch), leave it to the server. On a real merge the hook
raced it and reported "kept: remote ref does not exist" for a deleted branch.

Co-Authored-By: Claude Opus 5.5 <[email protected]>