pidState: count an unreaped Linux zombie as gone, so a mail lock held by an exited owner is reclaimable #713

Follow-up from #705 (Closes #693), noted out of scope by its builder.

What the fix does

pidState in packages/agent/src/lib/mail-lock.ts treats a pid as running when process.kill(pid, 0) succeeds. On Linux an exited process that has not been reaped yet (state Z in /proc/<pid>/stat) also passes that probe. #705 taught the test helper to count a zombie as gone. The production classifier still counts it as alive, so a mail lock whose owner exited but was never reaped stays held. That happens, for example, under a PID 1 that does not reap, such as a container started without an init process.

1. On Linux, pidState (and so processIsGone / the lock-owner check) reads /proc/<pid>/stat after a successful probe and classifies state Z as gone. EPERM and the non-Linux path are unchanged, and an unreadable stat file keeps today's answer.
2. A test spawns a child that exits without being reaped and asserts pidState returns gone on Linux. A second test asserts that a live child is still alive.
3. The doc comment on pidState states the zombie rule.

Priority: low. It only matters where the lock owner's parent does not reap its children.