test(integration): add git-refs push-queue semantics (D6 multi-worktree, D7 two-remote pin)

D6: the git-refs push queue lives in the shared git common dir, so sessions in two worktrees enqueue into ONE queue. A single pre-push from the main worktree pushes both worktrees' checkpoint refs, drains + compacts the queue, and prunes a stale entry (a queued ref with no local ref -> partitionLocalRefs). Adds an AddWorktree test helper (its own .entire/, shared common dir, offset session counter so session IDs don't collide in the shared session store).

D7: PIN the known two-remote queue-clearing gap (test plan decision D-2). The queue records only the ref, not the remote, so a push to remote A drains the queue and remote B never receives those refs. Asserts current behavior with a clear "flip when per-remote tracking lands" note. No production change.

Also resolves the COMMIT_EDITMSG scratch path via git rev-parse --absolute-git-dir so the shadow-commit test helpers work in a linked worktree (where .git is a file); identical to the prior path for normal checkouts.