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-status/scripts/status.sh

@@ -221,11 +221,10 @@ for s in d.get('herdr_sessions', []):
         'pane_cwd': pane.get('cwd'),
         'attach_command': s.get('attach_command'),
         'drift_classes': drift_by_name.get(name, []),
-        # Additive beyond the D8 example — needed by the M1 Detail Pane
-        # (pane pid / full launch command / start command / last banner text).
+        # Additive beyond the D8 example (pane pid / full launch command /
+        # last banner text).
         'pane_pid': pane.get('pid'),
         'cmd_full': pane.get('cmd_full'),
-        'start_command': s.get('start_command'),
         'last_visible_status': s.get('last_visible_status'),
     })