feat: update admission gate - entries refuse while an update runs (Issue #72, decision 8, U2)

The update skill publishes .mam/update-in-progress (pid + lstart, the
loop_lock.sh rules) while it holds .mam/update-admission.lock exclusively.
create, resume, run_loop.sh and delegate submit (decision 8) and stop and
orc-onboard (U2: a rollback would restore the registry without their write)
take that lock shared (lib.sh mam_update_admit, fd 7), refuse while the marker
is live or unknown, and hold the lock until their record is written. A stale
marker refuses nothing. lib_py.pane_lock acquire gains --shared and
--wait-env (MAM_UPDATE_ADMISSION_WAIT_S, default 120); create's wrapper path
closes fd 7 for the long-lived child. The exit-1 rows of create/resume/stop
name the refusal; skill_code_blocks.json pins follow the shifted lines.

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

.agents/skills/multi-agent-mux-stop/scripts/stop_session.sh

@@ -100,6 +100,11 @@ fi
 [ -n "$SESSION_NAME" ] || { echo "ERROR: --session required" >&2; usage; exit 2; }
 [ -f "$AGENT_SESSIONS_YAML" ] || { echo "ERROR: $AGENT_SESSIONS_YAML not found" >&2; exit 1; }
 
+# Issue #72 (U2): no registry write while an update runs - a rollback would
+# restore the registry without it. Held until the row is written (lib.sh
+# mam_update_admit).
+mam_update_admit || exit 1
+
 # Implement the purging-<session> file lock mechanism
 if [ "$PURGE" = "1" ]; then
   touch "$WORKSPACE_ROOT/.mam/purging-$SESSION_NAME"
@@ -453,6 +458,7 @@ if purge:
 else:
     print(f"updated: {name} status={target['status']}", flush=True)
 PYEOF
+mam_update_admit_release
 
 delegate_publish_event "$DELEGATE_JOB_ID" cancelled "session stopped by operator before job completion" "$DELEGATE_JOB_TOKEN" "$DELEGATE_JOB_ATTEMPT"