fix: attach_command from one builder; no kill/start_command; stop closes a row-less session (Issue #71)

- attach_command is `herdr --session <server> agent attach <herdr name>`
  (`default` written too), built by lib_py.agents.sanitize.attach_command;
  create, resume and reconcile all use it. kill_command and start_command
  are no longer written, and status --json drops start_command.
- The two raw kill-session hints (create exit 43, id_order's orphan) print a
  stop-skill command with --herdr-session instead.
- stop_session.sh closes a session that has no registry row, herdr only and
  without --agent, when herdr runs it inside this workspace; another
  workspace's session is refused (exit 5), a missing one is exit 1, one that
  survives kill-session is exit 6. The registry is not changed.
- Docs: only the SKILL.md lines these changes made wrong; the skill block
  pins follow. Tests that asserted the removed fields keep their intent
  (issue50: create's own row; issue21: no control-root path in
  attach_command; tier2: the new form and the absent fields). S11 pins the
  shim to 4243da5, which changed it on purpose.

Refs #71

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

.agents/skills/multi-agent-mux-resume/scripts/update_yaml_resumed.sh

@@ -116,6 +116,7 @@ atomic_dump_yaml "$AGENT_SESSIONS_YAML" \
   MAM_WS_LABEL="$MAM_WS_LABEL" MAM_WS_LABEL_EXPLICIT="${MAM_WS_LABEL_EXPLICIT:-0}" \
   ALREADY_RUNNING="$ALREADY_RUNNING" DROP_STALE_ID="$DROP_STALE_ID" \
   CMD_FULL_OPT="$CMD_FULL_OPT" <<'PYEOF'
+from lib_py.agents.sanitize import attach_command
 name = os.environ['SESSION_NAME']
 uuid = os.environ['UUID']
 agent = os.environ['AGENT']
@@ -147,9 +148,7 @@ if target is None:
         'herdr_workspace': wsl,
         'delegate_job_id': None,
         'pane': {'index': 0, 'pid': int(pane_pid) if pane_pid.isdigit() else 0, 'cmd': agent, 'cwd': ws_root},
-        'start_command': f'HERDR_SESSION_NAME={server_name} herdr agent attach {name}',
-        'attach_command': f'HERDR_SESSION_NAME={server_name} herdr agent attach {name}',
-        'kill_command': f'HERDR_SESSION_NAME={server_name} herdr kill-session -t {name}',
+        'attach_command': attach_command(server_name, name),
     }
     d.setdefault('herdr_sessions', []).append(target)
 else:
@@ -159,9 +158,7 @@ else:
         if is_explicit or not target.get('herdr_session'):
             target['herdr_session'] = sn
             target['herdr_server'] = sn
-            target['start_command'] = f'HERDR_SESSION_NAME={sn} herdr agent attach {name}'
-            target['attach_command'] = f'HERDR_SESSION_NAME={sn} herdr agent attach {name}'
-            target['kill_command'] = f'HERDR_SESSION_NAME={sn} herdr kill-session -t {name}'
+            target['attach_command'] = attach_command(sn, name)
     wsl = os.environ.get('MAM_WS_LABEL', '')
     ws_explicit = os.environ.get('MAM_WS_LABEL_EXPLICIT') == '1'
     if wsl and (ws_explicit or not target.get('herdr_workspace')):