feat(agent): add custom-agent self-updates with user isolation (#2713)

* feat(agent): add update_agent tool for in-chat custom-agent self-updates (#2616)

Custom agents had no built-in way to persist updates to their own SOUL.md /
config.yaml from a normal chat — `setup_agent` was only bound during the
bootstrap flow, so when the user asked the agent to refine its description
or personality, the agent would shell out via bash/write_file and the edits
landed in a temporary sandbox/tool workspace instead of
`{base_dir}/agents/{agent_name}/`.

Changes:
- New `update_agent` builtin tool with partial-update semantics (only the
  fields you pass are written) and atomic temp-file + os.replace writes so
  a failed update never corrupts existing SOUL.md / config.yaml.
- Lead agent now binds `update_agent` in the non-bootstrap path whenever
  `agent_name` is set in the runtime context. Default agent (no
  agent_name) and bootstrap flow are unchanged.
- New `<self_update>` system-prompt section is injected for custom agents,
  instructing them to use `update_agent` — and explicitly NOT bash /
  write_file — to persist self-updates.
- Tests: 11 new cases in `tests/test_update_agent_tool.py` covering
  validation (missing/invalid agent_name, unknown agent, no fields),
  partial updates (soul-only, description-only, skills=[] vs omitted),
  no-op detection, atomic-write safety, and AgentConfig round-tripping;
  plus 2 new cases in `tests/test_lead_agent_prompt.py` covering the
  self-update prompt section.
- Docs: updated backend/CLAUDE.md builtin tools list and tools.mdx
  (en/zh) with the new tool description.

* feat(agent): isolate custom agents per user

Store custom agent definitions under the effective user, keep legacy agents readable until migration, and cover API/tool/migration behavior with tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

* feat: consistent write/delete targets & add --user-id to migration

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_lead_agent_prompt.py

@@ -17,6 +17,18 @@ def _set_skills_cache_state(*, skills=None, active=False, version=0):
         prompt_module._enabled_skills_refresh_event.clear()
 
 
+def test_build_self_update_section_empty_for_default_agent():
+    assert prompt_module._build_self_update_section(None) == ""
+
+
+def test_build_self_update_section_present_for_custom_agent():
+    section = prompt_module._build_self_update_section("my-agent")
+
+    assert "<self_update>" in section
+    assert "my-agent" in section
+    assert "update_agent" in section
+
+
 def test_build_custom_mounts_section_returns_empty_when_no_mounts(monkeypatch):
     config = SimpleNamespace(sandbox=SimpleNamespace(mounts=[]))
     monkeypatch.setattr("deerflow.config.get_app_config", lambda: config)