row 305 (#1812): BUILD.md notes the SIGKILL leak and its cleanup (Filbert N1)

agents/gizmoduck/work/queue-305/BUILD.md

@@ -62,6 +62,9 @@ Commits: 541dee2b7 (draft), 3bce852d5 (presence half, mos-1445).
   - Before and after each run, record `docker network ls` filtered to `mosaic-test-`. The run's
     network must be absent after.
   - Grep `^skip` and name every skip.
+- A run killed by the timeout's SIGKILL skips the EXIT trap and leaks its one network. The name is
+  per-run under `mosaic-test-`, so the gate's after-listing finds it and removes it by its exact name
+  (Filbert N1; no code change).
 - If `docker info` works but there is no network to find, that's a first-run red. It goes to Sage
   for Mos, unfixed (mos-1445).