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/SKILL.md

@@ -94,11 +94,10 @@ lab-paper-pdf2md-creator-claude              default      running    alive  clau
       "job_id": null,
       "job_status": null,
       "pane_cwd": "/home/.../refer_landing_page",
-      "attach_command": "HERDR_SESSION_NAME=default herdr agent attach lab-landing-page-creator-claude",
+      "attach_command": "herdr --session default agent attach lab-landing-page-creator-claude",
       "drift_classes": [],
       "pane_pid": 12345,
       "cmd_full": "claude --dangerously-skip-permissions --session-id <uuid>",
-      "start_command": "...",
       "last_visible_status": "pinned"
     }
   ]