What was measured

Scratch bare repo, a clone, and a second clone standing in for another filing host. Sequence, all of it production shape:

the other host files backlog/<slug>.md and pushes;
this clone fetches and checks out that sha detached — the stub is in the worktree, so stub_backlog_path() finds it and the body reads fine;
the close writes done/<slug>.md and unlinks the backlog copy;
Clone.publish() runs git checkout --quiet master — and this clone's master predates the filing commit, so the path is neither present nor tracked;
git add -- devdocs/progress/backlog/<slug>.md devdocs/progress/done/<slug>.md → fatal: pathspec ... did not match any files, exit 128.

sh() raises, and nothing between close_stub_tickets and the cycle catches it, so every other close queued in the same call is lost too.

Why it is prio 30 and not higher

It cannot produce a wrong verdict, and it is loud. It also needs a filer that is not this host: publish() runs on essentially every cycle and leaves the clone's branch at origin, so the window is small with one watcher. plexus and seven are both RETIRED, so today there is exactly one. Re-rank this up if a second watcher host is enrolled — the xeon enrolment plan would do it.

Fix shape, and why it is not patched here

git add failing on a pathspec that matches nothing is the correct behaviour of the instrument; what is wrong is that publish() is handed a path its branch cannot know about. Candidates, none obviously right:

have publish() drop paths that are neither present nor tracked, with a printed reason — cheap, but it makes a real staging bug silent, which is the thing publish()'s current shape is deliberately loud about;
have close_stub_tickets() refuse a stub whose path is absent from the branch tip and say so — narrower, one extra git call per close;
have the close pull before it decides, so the branch carries what it read.

That is a contract question about publish(), which every verdict goes through. It wants deciding rather than guessing.

Repro

tools/twatch_close_stubs_devtest.py case 8 covers the sibling (duplicate) bug. This one is NOT guarded — a guard would have to assert a raise, and the fix will change what happens instead, so the guard belongs with the fix.