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_issue21_worktree_prereqs.py

@@ -581,7 +581,7 @@ def test_t3_1_create_persists_and_forwards_a_foreign_binding(mam_sandbox, mock_h
     for k, v in zip(FORWARDED_ENV, list(_binding(tmp).values())[:3]):
         assert f"{k}={v}" in flags, flags
     assert "WORKSPACE_ROOT=" not in row["pane"]["cmd_full"]
-    assert "WORKSPACE_ROOT=" not in row["start_command"]
+    assert "WORKSPACE_ROOT=" not in row["attach_command"]
     _assert_no_env_in_agent_args(mock_herdr)