fix(timeline): exclude attachments from canonical prompt text

Structured provider prompts can include hidden history attachments before the user's text. Persist only plain prompt blocks so authoritative user messages match what the composer displayed while provider input remains unchanged.

packages/server/src/server/agent/agent-manager.test.ts

@@ -8183,9 +8183,20 @@ test("authoritative timeline includes provider-emitted submitted user prompt", a
       workspaceId: undefined,
     });
 
-    await manager.runAgent(snapshot.id, "hello from composer", {
-      clientMessageId: "msg-client-1",
-    });
+    await manager.runAgent(
+      snapshot.id,
+      [
+        {
+          type: "text",
+          mimeType: "text/plain",
+          contextKind: "chat_history",
+          title: "Chat history",
+          text: "<chat-history-summary>hidden fork history</chat-history-summary>",
+        },
+        { type: "text", text: "hello from composer" },
+      ],
+      { clientMessageId: "msg-client-1" },
+    );
 
     const liveEvents = events.filter(
       (event): event is Extract<AgentManagerEvent, { type: "agent_stream" }> =>

packages/server/src/server/agent/agent-manager.ts

@@ -95,7 +95,7 @@ function submittedPromptText(prompt: AgentPromptInput): string {
     return prompt;
   }
   return prompt
-    .flatMap((block) => (block.type === "text" ? [block.text] : []))
+    .flatMap((block) => (block.type === "text" && !("mimeType" in block) ? [block.text] : []))
     .join("\n")
     .trim();
 }