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_issue63_same_cwd_ids.py

@@ -1090,7 +1090,9 @@ def test_id_order_dead_owner_marker_blocks_while_its_unregistered_herdr_session_
         assert marker_a.exists()
         assert ("srv-x", "orphan-a-agy") in probes
         err = capsys.readouterr().err
-        assert "HERDR_SESSION_NAME=srv-x herdr kill-session -t orphan-a-agy" in err
+        # Issue #71: the hint is a stop-skill command naming the server
+        assert "bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh --session orphan-a-agy --herdr-session srv-x" in err
+        assert "kill-session" not in err
 
         # No authoritative answer from herdr -> kept and blocks as well
         unknown_sessions.add("orphan-a-agy")
@@ -1137,7 +1139,7 @@ def test_id_order_dead_owner_marker_blocks_while_its_unregistered_herdr_session_
         assert probes == [("", "same-agy")]
         assert json.loads(same.read_text())["owner_pid"] == live.pid
         err = capsys.readouterr().err
-        assert "its herdr session is alive" in err and "kill-session -t same-agy" in err
+        assert "its herdr session is alive" in err and "bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh --session same-agy --herdr-session " in err
         unknown_sessions.add("same-agy")
         assert wait_same() == 1
         assert json.loads(same.read_text())["owner_pid"] == live.pid
@@ -1188,7 +1190,7 @@ def test_id_order_create_waits_for_orphan_herdr_session_of_a_killed_create(
 
         res = create_b()
         assert res.returncode == 1, res.stdout + res.stderr
-        assert name_a in res.stderr and "kill-session -t " + name_a in res.stderr
+        assert name_a in res.stderr and "bash .agents/skills/multi-agent-mux-stop/scripts/stop_session.sh --session " + name_a + " --herdr-session orphan-ws" in res.stderr
         assert "its herdr session is alive" in res.stderr
         assert name_b not in _read_rows(mam_sandbox)
         assert name_b not in json.loads(Path(mock_herdr).read_text()).get("agents", {})