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-resume/SKILL.md

@@ -75,11 +75,11 @@ bash .agents/skills/multi-agent-mux-resume/scripts/resume_session.sh --workspace
 | Exit | Meaning |
 |---|---|
 | 0 | Resumed; or already running, so only the row was updated (no attach); or `--dry-run` |
-| 1 | From the checks before spawn, nothing was changed: no saved session for claude/grok (use `multi-agent-mux-create` first), the row's id is also claimed by another row, a same-cwd Class A session did not record its id within `MAM_ID_ORDER_WAIT_S` (default 600), or agent binary missing/not executable. **The YAML update after spawn (`update_yaml_resumed.sh`) also exits 1** (YAML not found) |
+| 1 | From the checks before spawn, nothing was changed: no saved session for claude/grok (use `multi-agent-mux-create` first), the row's id is also claimed by another row, a same-cwd Class A session did not record its id within `MAM_ID_ORDER_WAIT_S` (default 600), agent binary missing/not executable, or an update of the workspace is in progress (`multi-agent-mux-update`). **The YAML update after spawn (`update_yaml_resumed.sh`) also exits 1** (YAML not found) |
 | 2 | From the checks before spawn, nothing was changed: bad arguments, `--workspace` differs from the original cwd or it no longer exists, control-plane binding refused, whitespace in the control-plane path. **The YAML update after spawn also exits 2** (bad arguments) |
 | other | A later step failed (herdr spawn, start-up dialogs, YAML update) and its code is passed through. resume does not roll back |
 
-A non-zero exit after the spawn can leave the herdr session running while the row is still `stopped`. Before retrying or creating a new session, check with `multi-agent-mux-status`. The `ERROR:` line on stderr tells the two apart: the pre-spawn checks' messages are in `resume_session.sh` (lines 50-243).
+A non-zero exit after the spawn can leave the herdr session running while the row is still `stopped`. Before retrying or creating a new session, check with `multi-agent-mux-status`. The `ERROR:` line on stderr tells the two apart: the pre-spawn checks' messages are in `resume_session.sh` (lines 50-259).
 
 ## Original cwd only (Issue #21 phase 2)