feat: add IM channels for Feishu, Slack, and Telegram (#1010)

* feat: add IM channels system for Feishu, Slack, and Telegram integration

Bridge external messaging platforms to DeerFlow via LangGraph Server with
async message bus, thread management, and per-channel configuration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address review comments on IM channels system

Fix topic_id handling in store remove/list_entries and manager commands,
correct Telegram reply threading, remove unused imports/variables, update
docstrings and docs to match implementation, and prevent config mutation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* update skill creator

* fix im reply text

* fix comments

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

backend/tests/test_reflection_resolvers.py

@@ -8,6 +8,7 @@ from src.reflection.resolvers import resolve_variable
 
 def test_resolve_variable_reports_install_hint_for_missing_google_provider(monkeypatch: pytest.MonkeyPatch):
     """Missing google provider should return actionable install guidance."""
+
     def fake_import_module(module_path: str):
         raise ModuleNotFoundError(f"No module named '{module_path}'", name=module_path)
 
@@ -38,6 +39,8 @@ def test_resolve_variable_reports_install_hint_for_missing_google_transitive_dep
     message = str(exc_info.value)
     # Even when a transitive dependency is missing, the hint should still point to the provider package.
     assert "uv add langchain-google-genai" in message
+
+
 def test_resolve_variable_invalid_path_format():
     """Invalid variable path should fail with format guidance."""
     with pytest.raises(ImportError) as exc_info: