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>

tests/test_tier1_unit.py

@@ -217,7 +217,11 @@ def test_stop_is_already_stopped_not_found(mam_sandbox):
     assert res.returncode == 1
 
 def test_stop_session_invalid_agent_suffix(mam_sandbox):
-    """Test stop_session.sh fails when agent cannot be inferred from session name."""
+    """Test stop_session.sh fails when agent cannot be inferred from session name.
+    Issue #71: a session with no row needs no agent kind (stop closes it herdr
+    only), so the registered row here carries no agent, suffix or pane.cmd."""
+    (mam_sandbox / ".mam" / "agent-sessions.yaml").write_text(
+        "herdr_sessions:\n- name: bad-session-name\n  status: running\n  pane: {}\n")
     script_path = mam_sandbox / "skills" / "multi-agent-mux-stop" / "scripts" / "stop_session.sh"
     res = subprocess.run(["bash", str(script_path), "--session", "bad-session-name"], capture_output=True, text=True)
     assert res.returncode == 2