docs: fill all TBD documentation pages and add new harness module pages

Agent-Logs-Url: https://github.com/bytedance/deer-flow/sessions/ff389ed8-31c9-430c-85ff-cc1b52b8239c

Co-authored-by: foreleven <4785594+foreleven@users.noreply.github.com>

frontend/src/content/en/application/agents-and-threads.mdx

@@ -1,3 +1,5 @@
+import { Callout, Cards, Steps } from "nextra/components";
+
 # Agents and Threads
 
 DeerFlow App supports multiple named agents and maintains conversation state across sessions through threads and checkpointing.
@@ -104,11 +106,10 @@ checkpointer:
 ```
 
 <Callout type="info">
-  DeerFlow App uses the <code>checkpointer</code> setting in{" "}
-  <code>config.yaml</code> to persist thread state through the Gateway runtime
-  (via <code>make_checkpointer()</code> in
-  <code>deerflow.agents.checkpointer.async_provider</code>). Configure this
-  section if you want threads to survive process restarts.
+  The LangGraph Server manages its own state separately. The
+  <code>checkpointer</code> setting in <code>config.yaml</code> applies to the
+  embedded <code>DeerFlowClient</code> (used in direct Python integrations), not
+  to the LangGraph Server deployment used by DeerFlow App.
 </Callout>
 
 ### Thread data storage