Flaky: test_skill_listing_cost supplied_mtime test catches a stray config.json stat (Linux + Windows) #21079

What

test/test_skill_listing_cost.py::TestListingStaysCheapOnTheEventLoop::test_a_supplied_mtime_skips_the_frontmatter_cache_stat failed on 3 unrelated branches. It fails on both Linux and Windows, always in shard 3:

• https://github.com/kirodotdev/KiroCrew/actions/runs/38674134485 (fix/20289-n-bin-acp-discovery, Linux 3.12, 2026-10-15)
• https://github.com/kirodotdev/KiroCrew/actions/runs/38799180897 (fix/15515-agent-spec-unreadable-skip, Linux 3.12, 2026-10-15)
• https://github.com/kirodotdev/KiroCrew/actions/runs/38826927514 (fix/prompt-optimize-cancel, Windows, 2026-10-16)

AssertionError: assert ['/tmp/pytest.../config.json'] == []

Why it's a flake

None of these branches touch skill listing. The test asserts that no stat happens on the event loop. The stat it catches is of a config.json under the pytest tmp dir, not a skill frontmatter file. That looks like a stat from some other path, like a config load in a background or earlier-scheduled task, landing inside the test's observation window. If so, the result depends on what else the worker ran first (determinism class 4, order dependence).

Likely fix direction

Narrow the stat spy to the frontmatter-cache paths the test is about. Or make sure nothing else on the loop loads config.json during the window, for example by pinning the config load before the spy starts.

(Filed by the CI flake patrol.)