Codex gate found a SILENT regression: the cache returned on a zero-I/O fast path
while inside the 300s TTL, so _skill_tree_max_mtime_ns never ran during the TTL
window and an out-of-band (CLI/git) skill create/edit/delete was invisible for up
to 5 minutes — WORSE than master's ~8s stale window. Restructured so the cheap
stat-only mtime probe runs on EVERY call (prompt change detection); the TTL now
only bounds the expensive SKILL.md recompute. Unchanged mtime still serves cached
+ refreshes TTL with no recompute (the perf win is preserved). Updated 2 tests
that encoded the flawed zero-I/O design + added a within-TTL-change regression
(proven non-vacuous).
Co-authored-by: Rod Boev <rod.boev@gmail.com>
