Two related failure modes after a crashed/interrupted fetch on a shallow
clone (git clone --depth 1 installs):
1. STALE LOCK WEDGES EVERY FETCH. A killed fetch can leave .git/shallow.lock
behind; every later 'git fetch' then fails with 'Unable to create
.../shallow.lock: File exists'. 'hermes update --check' reported a hard
fetch failure, and the passive banner check swallowed the exception and
compared stale refs. Add hermes_cli.gitlock.clear_stale_git_locks(), a
guarded sweep (age + git-process check so a live fetch is never yanked)
wired into the check path, the apply path, and the banner's passive check.
2. SHALLOW TIP-SHA COMPARE FALSE-POSITIVES. On a shallow clone the check
cannot count commits, so it compares tip SHAs. Local cherry-picks on top
of the remote tip (e.g. re-applied local patches) make HEAD differ from
origin/main even though HEAD already contains it — a false 'update
available' banner. Add hermes_cli.gitlock.is_ancestor_of_head() and use
'git merge-base --is-ancestor' in the CLI check and banner paths before
reporting an update. Mirror in the desktop (update-count.ts gains an
isAncestor input; main.ts probes merge-base --is-ancestor).
Tests: tests/test_gitlock.py (9) covering stale/young/no-lock/no-repo sweeps
and ancestry true/false; update-count.test.ts +3 for the isAncestor path.
