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_issue42_skill_blocks.py

@@ -217,7 +217,7 @@ def test_literal_exit_rule_on_base_scripts():
     """The counting rule: heredocs and comments skipped, so create's header `exit 4`
     and stop's `sys.exit(7)` are not counted."""
     assert literal_exit_codes("multi-agent-mux-resume/scripts/resume_session.sh") == {"0", "1", "2"}
-    assert literal_exit_codes("multi-agent-mux-stop/scripts/stop_session.sh") == {"0", "1", "2", "3", "4"}
+    assert literal_exit_codes("multi-agent-mux-stop/scripts/stop_session.sh") == {"0", "1", "2", "3", "4", "5", "6"}
     assert literal_exit_codes("multi-agent-mux-create/scripts/create_session.sh") == {"0", "1", "2", "3", "42", "43", "44", "45"}