Why: relaxAndRemove cleared the per-exec id directory but left the -ws
parent behind, so every conversation accumulated an empty husk - the
live probe caught it in the workspace listing.
Evidence: go test ./... green; TestOverlayCleansShellDir pins the
behavior with a real bwrap roundtrip.
Impact: One extra os.Remove of the (now empty) parent; failures are
best-effort as before.
Refs: Dev task feature/agent-s0-workspace-overlay (fix-up of #135).
weichen