gbrainAvailable() spawned gbrain --version under a 500ms budget; a cold CLI
start on a loaded machine blew the timeout, misclassified gbrain as missing,
and every skill session silently ran brainless — plus the per-query re-probe
burned 3x the budget before any real work. Replaced with a memoized
stat-based PATH scan (PATHEXT-aware on Windows) and made the query timeout
overridable via GSTACK_BRAIN_TIMEOUT_MS for loaded CI environments.
Also picks up the fork's manifest-filter coverage (#1687 shape) against the
fake-gbrain harness — passes against our existing filter support.
Ported from time-attack/gstack (GStack 2).
Co-authored-by: Sina Matian <sina@time-attack.dev>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
