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-loop/scripts/run_loop.sh

@@ -244,6 +244,9 @@ log_error() {
   echo -e "\033[1;31m[✗]\033[0m $1"
 }
 
+# Issue #72: no loop starts while an update runs; held until the loop marker
+# is published (lib.sh mam_update_admit), released before the loop body runs.
+mam_update_admit "$MAM_REAL_ROOT" || exit 1
 _mam_acquire_rc=0
 mam_acquire_loop_lock "$MAM_LOOP_MARKER" || _mam_acquire_rc=$?
 case "$_mam_acquire_rc" in
@@ -262,6 +265,7 @@ case "$_mam_acquire_rc" in
     ;;
 esac
 trap _mam_release_guard EXIT INT TERM HUP
+mam_update_admit_release
 rm -f "$MAM_REAL_ROOT/.agents/skills/multi-agent-mux-delegate-job/multi-agent-mux-delegate-job".*.tmp 2>/dev/null || true
 
 # --all-reviewer silently takes precedence over an explicit --reviewer list;