fix(agent-threads): hide prompt reference directives in thread titles

Add a shared visiblePromptText helper and strip reference tokens,
image-generator directives, and model-selection lines from agent thread
summaries and the assistant panel header, so titles show the user's
actual prompt instead of internal directives. Covered by a server test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/application/agent_threads_test.go

@@ -44,6 +44,44 @@ func TestAgentThreadsOnlyListsAgentConversationThreads(t *testing.T) {
 	}
 }
 
+func TestAgentThreadsSummaryHidesPromptReferenceDirectives(t *testing.T) {
+	ctx := context.Background()
+	repo := memory.NewProjectRepository()
+	service := NewDesignService(repo, nil, nil, nil, nil)
+	now := time.Now()
+	project := design.Project{
+		ID:           "project-agent-reference-history",
+		Title:        "Agent history",
+		Status:       design.StatusReady,
+		LastThreadID: "thread-agent",
+		UpdatedAt:    now,
+		Messages: []design.Message{
+			{
+				ID:        "agent-user",
+				Role:      "user",
+				Type:      "user",
+				Content:   "Reference image 1 (image): http://localhost:19000/canvas-assets/reference.png 灰裤子拆成单件衣服，为了后面电商配图。\nSelected model: GPT Image 2",
+				ThreadID:  "thread-agent",
+				CreatedAt: now,
+			},
+		},
+	}
+	if err := repo.Save(ctx, project); err != nil {
+		t.Fatal(err)
+	}
+
+	threads, err := service.AgentThreads(ctx, project.ID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(threads) != 1 {
+		t.Fatalf("expected one agent thread, got %#v", threads)
+	}
+	if threads[0].Text != "灰裤子拆成单件衣服，为了后面电商配图。" {
+		t.Fatalf("expected visible prompt text only, got %q", threads[0].Text)
+	}
+}
+
 func TestProjectScopedToAgentThreadKeepsOnlyCurrentConversationContext(t *testing.T) {
 	now := time.Now()
 	project := design.Project{