parallel instance check

I usually have several agent sessions working the same repository at once, so no session may assume it's alone. Before any commit, branch, or push: fetch, scan recent remote activity, and check whether the files about to change just moved under someone else's hands.

I once pushed a verified fix to a fresh branch while another session had already landed the same commit on main — dead-on-arrival duplication a five-second fetch would have caught. Shared trees need the check every time.

If the fix already landed on main from another session, reconcile to it instead of pushing a duplicate. Commits use an explicit pathspec so a parallel session's staged files never get swept in. Shared coordination files are append-only, one line per event, so concurrent writes merge instead of clobbering.

The whole method is downloadable and adaptable — rules/parallel-instance-check.md inside the zip.