feat(issue-21): persist and forward the control-plane binding in create/resume/stop; status and reconcile honor MAM_JOBS_DIR

A session row now records the control plane it belongs to, and the pane
gets it explicitly instead of relying on env inheritance.

- lib_py/control_plane.py (+ thin lib.sh wrappers mam_control_plane_json /
  mam_control_plane_prefix): the binding is WORKSPACE_ROOT,
  AGENT_SESSIONS_YAML, MAM_JOBS_DIR, DELEGATE_JOB_LOGS_DIR as absolute paths.
- create: the binding is always persisted on the row (`control_plane`) and
  forwarded to the pane only when it differs from what --workspace alone
  implies, as leading K=V tokens that lib.sh's new-session already turns into
  --env (fresh pane) or `env K=V` (reused idle pane). pane.cmd_full and
  start_command stay unprefixed. A binding with whitespace that would have to
  be forwarded is refused with exit 2 (lib.sh splits the flags unquoted); the
  ~/.local/bin wrapper path records the binding but warns it is not forwarded.
- resume: the row's binding wins over the caller's env (a difference is
  reported on stderr) and is forwarded again; a legacy row without one uses
  the caller's binding and update_yaml_resumed.sh records it.
- stop: registry order MAM_JOBS_DIR > the row's mam_jobs_dir >
  $WORKSPACE_ROOT/.mam/jobs > .mam/jobs, exported for delegate_publish_event.
- status: both get_job_status copies look in MAM_JOBS_DIR, the row's
  registry and the matching audit logs before the old candidates.
- reconcile: MAM_JOBS_DIR wins in the subscribe heredoc and is passed in.

Tests T3-0..T3-8 (mock herdr: forwarding is verified at the herdr-call level
only; real pane env propagation is left to item 7). T3-7 is a static guard.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017R3Eg7MEpSWnm1oB9ZPP6G

.agents/skills/multi-agent-mux-monitor/scripts/reconcile.sh

@@ -100,7 +100,7 @@ if [ "$SUBSCRIBE" = "1" ]; then
   # The MQTT subscribe loop exits 3 to signal "broker unavailable → poll instead".
   set +e
   YAML_PATH="$AGENT_SESSIONS_YAML" HOME_DIR="$HOME_DIR" CLAUDE_PROJECT_DIR="$CLAUDE_PROJECT_DIR" LOCAL_BIN="$LOCAL_BIN" \
-    WORKSPACE_ROOT="$WORKSPACE_ROOT" SUB_TIMEOUT="$SUB_TIMEOUT" SUB_IDLE_TIMEOUT="$SUB_IDLE_TIMEOUT" \
+    WORKSPACE_ROOT="$WORKSPACE_ROOT" MAM_JOBS_DIR="${MAM_JOBS_DIR:-}" SUB_TIMEOUT="$SUB_TIMEOUT" SUB_IDLE_TIMEOUT="$SUB_IDLE_TIMEOUT" \
     SKILLS_DIR="$SKILLS_DIR" LIB_SH="$LIB_SH" \
     "$PYBIN" - <<'PYEOF' &
 import os, sys, json, time, subprocess
@@ -236,7 +236,9 @@ def on_message(_client, _userdata, msg):
         if not jid or not event:
             return
         
-        if workspace_root:
+        if os.environ.get('MAM_JOBS_DIR'):
+            registry_dir = os.environ['MAM_JOBS_DIR']  # Issue #21: MAM_JOBS_DIR wins
+        elif workspace_root:
             registry_dir = os.path.join(workspace_root, '.mam', 'jobs')
         else:
             yaml_dir = os.path.dirname(yaml_path) if yaml_path else ""