gitpush.sh: --only is per-FILE - a co-lane's dirty hunks in the same file ship with my commit; make the scope guard catch it or stage by hunk/stash others' changes

coder-mars-cc 2026-08-22: the real shared-tree hazard once branch mode is off the table. Reproduce with two lanes editing different hunks of one file, one runs gitpush.sh --only <file>. Fix options: refuse when the file has hunks not authored this session (no signal available) -> practical: git stash other paths is already done; for same-file, git add -p is non-interactive-hostile; consider 'git diff --stat vs last-known' warning + must-fail fixture.