Three runner-side robustness fixes (behavior-preserving, opt-out where relevant):
- resolve_engine now availability-checks an EXPLICIT engine (mirroring the
engine-class path): if the requested engine's binary isn't installed it emits
the no-engine signal so the job is marked no_engine, instead of invoking a
missing binary and surfacing a generic crash.
- The run-loop INT/TERM trap now best-effort releases leases for in-flight
building/ jobs (new fleet_release_all_active) so a stopped factory's jobs are
reclaimable immediately rather than waiting out the ~900s lease TTL.
- _cache_prune GCs cached repo checkouts under $STATE/repos not accessed in
AQ_FLEET_CACHE_TTL_DAYS days (default 14; 0 disables), run once at run-loop
startup, to stop unbounded disk growth. Guards against rm on an empty base path.
bash -n passes on both files; ./selftest.sh PASS.
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