test(resume-arm): make the worktree refusal test exercise the refusal it is named for

[jaylfc]

Closes tsk-3m3irr, the follow-up carded when #369 was merged. Test-only: scripts/resume_arm_time.py is untouched, because the guard was already correct.

The defect

_validate_helper_path() refuses two things, a helper path under a temp directory and one inside a linked git worktree. Only the first was covered.

test_guard_refuses_linked_worktree never called main() and never asserted a refusal. It called the detector and checked it returned a string:

result = resume_arm_time._is_in_linked_worktree(str(target))
assert result == str(fake_wt)

The name promises guard_refuses. The body proves a path-inspection function returns a string, so anyone reading the suite sees a test named for the behaviour and reasonably assumes the behaviour is covered.

Why a whole-suite count could never have found this

Killing the entire guard fails exactly one test, and one failure reads as coverage. One honest test absorbed a whole-feature mutant and hid a vacuous sibling behind it. The mutant has to be aimed at the individual branch, leaving the detector intact so the existing test still has its function to call:

wt = _is_in_linked_worktree(path)
if False:  # MUTANT: worktree refusal removed

mutant	before	after
worktree refusal only, detector intact	9 passed	1 failed, 10 passed on test_guard_refuses_linked_worktree
whole guard disabled	1 failed, 8 passed	2 failed, 9 passed

Both "before" figures reproduce the card's recorded measurements exactly. Under the first mutant the real CLI goes on emitting four crontab lines pinning a path that vanishes when the worktree is removed, which is precisely what the guard exists to prevent.

The trap in the fix, which is the part worth reading

The obvious fix does not work, and it fails in a way that looks like success.

The two refusals are checked in order, temp first. tmp_path is itself under a temp root. So a fixture built there trips the temp refusal and never reaches the branch the test is named for. A test that only asserted SystemExit would pass, would survive the whole-guard mutant, and would still not notice the worktree refusal being deleted. It would be the same vacuity in a new costume.

So the test neutralises _is_under_temp deliberately and is worthless without it, which the docstring and the changelog both state outright. It then asserts the message names the worktree, names the fixture directory, and specifically that it is not the sibling refusal:

assert "is inside a git worktree" in message
assert str(fake_wt) in message
assert "temp directory" not in message   # not the sibling refusal wearing this test's name

Also added

The detector assertion is kept, under the honest name test_is_in_linked_worktree_detects_git_file, plus test_is_in_linked_worktree_ignores_the_main_worktree. The second pins behaviour that must not change: in the main worktree .git is a directory and the guard must keep not firing, or arming stops working on the live box. That is deliberate scope, not an oversight.

Verification

full suite:  1648 passed, 12 skipped     (baseline 1646 post-#370, exactly +2)
git diff origin/master --stat:  2 files, 50 insertions, 1 deletion
scripts/resume_arm_time.py:     unchanged, confirmed pristine after both mutants
git grep conflict markers:      none
deleted-symbols-guard / normalise-handle-gate / witness-gate:  all clean
tail -c 1 changelog.d/tsk-3m3irr-worktree-refusal-test.md | xxd -p  ->  0a
em dashes in both changed files:  0

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Reviewing files that changed from the base of the PR and between 35c4205 and 6810b36.

📒 Files selected for processing (2)

• changelog.d/tsk-3m3irr-worktree-refusal-test.md
• tests/test_resume_arm_time.py

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Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• tests/test_resume_arm_time.py
• changelog.d/tsk-3m3irr-worktree-refusal-test.md

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_{Reviewed by step-3.7-flash · Input: 56.2K · Output: 11.4K · Cached: 144.4K}