fix(reset): refuse every registration during a reset and purge only ids owned by the row's folder (#78)

Review round 2 (codex):
- R1: register_job reads .mam/reset-in-progress under the registry lock and
  refuses while a reset of the workspace owning <ws>/.mam/jobs is live, so the
  public registry.py register CLI and the API no longer bypass the gate. The
  reset's precheck takes that lock once after publishing its marker, so a
  registration is either seen by it or refused. The reset's own create passes
  MAM_RESET_OWNER_PID; the remote runner refuses the dispatch.
- R2: before each purge the reset re-runs stop's revalidate check
  (verify_session_uuid) for a post-discovered id against the row's folder and
  skips the target (id not verified) when it does not verify.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.agents/skills/multi-agent-mux-delegate-job/registry.md

@@ -141,8 +141,9 @@ $PY scripts/registry.py status --job <id> --set completed          # set status
 $PY scripts/registry.py pick   --agent-session herdr:claude         # claim → running
 ```
 
-Exit codes: `0` ok, `1` not found / bad status, `3` (`pick`) no pending job for
-that session.
+Exit codes: `0` ok, `1` not found / bad status / (`register`) a reset of the
+workspace owning `<ws>/.mam/jobs` is in progress (Issue #78), `3` (`pick`) no
+pending job for that session.
 
 ---