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/lib_py/agents/id_order.py

@@ -341,7 +341,8 @@ def wait(cwd: str, agent: str, session: str, owner_pid: int,
                     print(
                         f"ERROR: {m['session']}'s create/resume exited before recording it, "
                         f"and {seen}; close it with: "
-                        f"HERDR_SESSION_NAME={srv} herdr kill-session -t {m['session']}",
+                        f"bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh "
+                        f"--session {m['session']} --herdr-session {srv}",
                         file=sys.stderr, flush=True,
                     )
             return 1