feat(FN-1975): stream chat session updates through SSE

- Emit chat:session:updated from ChatStore when message deletion mutates a session and cover deleteMessage false returns
- Pass ChatStore into SSE setup and forward chat session update events to connected clients
- Update useChat to consume SSE updates in real time with safer EnrichedChatSession typing
- Add core and dashboard tests for chat-store emissions, SSE forwarding, route wiring, and hook behavior

packages/core/src/__tests__/chat-store.test.ts

@@ -555,6 +555,31 @@ describe("ChatStore", () => {
         expect(store.getMessages(session2.id)).toHaveLength(1);
         expect(store.getMessages(session2.id)[0].content).toBe("Session 2");
       });
+
+      it("updates the parent session's updatedAt timestamp", async () => {
+        const session = createTestSession(store);
+        store.addMessage(session.id, { role: "user", content: "Hello" });
+        const originalUpdatedAt = store.getSession(session.id)!.updatedAt;
+
+        await new Promise((r) => setTimeout(r, 5));
+
+        const msg = store.addMessage(session.id, { role: "assistant", content: "Reply" });
+        const afterAddUpdatedAt = store.getSession(session.id)!.updatedAt;
+
+        await new Promise((r) => setTimeout(r, 5));
+
+        store.deleteMessage(msg.id);
+
+        const afterDeleteUpdatedAt = store.getSession(session.id)!.updatedAt;
+
+        // The updatedAt should be newer after adding and after deleting
+        expect(new Date(afterAddUpdatedAt).getTime()).toBeGreaterThan(
+          new Date(originalUpdatedAt).getTime(),
+        );
+        expect(new Date(afterDeleteUpdatedAt).getTime()).toBeGreaterThan(
+          new Date(afterAddUpdatedAt).getTime(),
+        );
+      });
     });
   });
 
@@ -627,6 +652,20 @@ describe("ChatStore", () => {
       expect(handler).toHaveBeenCalledWith(message.id);
     });
 
+    it("deleteMessage emits chat:session:updated for the parent session", () => {
+      const handler = vi.fn();
+      store.on("chat:session:updated", handler);
+
+      const session = createTestSession(store);
+      const message = store.addMessage(session.id, { role: "user", content: "Hello" });
+      handler.mockClear();
+
+      store.deleteMessage(message.id);
+
+      expect(handler).toHaveBeenCalledTimes(1);
+      expect(handler.mock.calls[0][0].id).toBe(session.id);
+    });
+
     it("deleteMessage does NOT emit for non-existent message", () => {
       const handler = vi.fn();
       store.on("chat:message:deleted", handler);
@@ -636,6 +675,15 @@ describe("ChatStore", () => {
       expect(handler).not.toHaveBeenCalled();
     });
 
+    it("deleteMessage does NOT emit chat:session:updated for non-existent message", () => {
+      const handler = vi.fn();
+      store.on("chat:session:updated", handler);
+
+      store.deleteMessage("msg-nonexistent");
+
+      expect(handler).not.toHaveBeenCalled();
+    });
+
     it("archiveSession emits chat:session:updated", () => {
       const handler = vi.fn();
       store.on("chat:session:updated", handler);

packages/core/src/chat-store.ts

@@ -387,9 +387,23 @@ export class ChatStore extends EventEmitter<ChatStoreEvents> {
     const existing = this.getMessage(id);
     if (!existing) return false;
 
+    const sessionId = existing.sessionId;
+    const now = new Date().toISOString();
+
     this.db.prepare("DELETE FROM chat_messages WHERE id = ?").run(id);
+
+    // Update the parent session's updatedAt timestamp
+    this.db.prepare("UPDATE chat_sessions SET updatedAt = ? WHERE id = ?").run(now, sessionId);
+
     this.db.bumpLastModified();
     this.emit("chat:message:deleted", id);
+
+    // Emit session:updated for the parent session
+    const updatedSession = this.getSession(sessionId);
+    if (updatedSession) {
+      this.emit("chat:session:updated", updatedSession);
+    }
+
     return true;
   }
 }

packages/core/src/chat-types.ts

@@ -45,6 +45,18 @@ export interface ChatSession {
  */
 export type ChatSessionSummary = ChatSession;
 
+/**
+ * Chat session enriched with last message preview data.
+ * The server enriches sessions with lastMessagePreview and lastMessageAt
+ * by fetching the most recent message for each session.
+ */
+export type EnrichedChatSession = ChatSession & {
+  /** Preview of the last message in the session (truncated to 100 chars) */
+  lastMessagePreview?: string;
+  /** Timestamp of the last message in the session */
+  lastMessageAt?: string;
+};
+
 /** A parsed @ mention of an agent in a chat message */
 export interface ChatMention {
   agentId: string;

packages/core/src/index.ts

@@ -588,6 +588,7 @@ export type {
   ChatMessageRole,
   ChatSession,
   ChatSessionSummary,
+  EnrichedChatSession,
   ChatMention,
   ChatMessage,
   ChatMessageCreateInput,