feat(piagent): bypass planner for direct image generation modes

For image / pure-image / image-generator / image-action modes, skip the
task-planning round trip and hand the prompt straight to the image model,
deriving the requested image count from the composer settings. Also
normalize completed image-generator nodes back into regular image nodes
(clearing the image-generator layer role and defaulting Tone to "visual")
when merging generated results and when restoring finished nodes, so a
finished generator node is not re-appended as a duplicate.

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

server/internal/application/image_generator_target_test.go

@@ -50,6 +50,44 @@ func TestMergeGeneratedNodesIntoImageGeneratorTargetUsesGeneratedSizeAndKeepsCen
 	}
 }
 
+func TestMergeGeneratedNodesIntoCompletedImageGeneratorTargetDoesNotReappend(t *testing.T) {
+	nodes := []design.Node{
+		{
+			ID:        "generator-1",
+			Type:      design.NodeTypeImage,
+			Title:     "旧图",
+			Content:   "https://example.com/old.png",
+			Tone:      "visual",
+			Status:    "success",
+			LayerRole: "",
+			Width:     360,
+			Height:    240,
+		},
+	}
+	generated := []design.Node{
+		{
+			ID:        "generator-1",
+			Type:      design.NodeTypeImage,
+			Title:     "新图",
+			Content:   "https://example.com/new.png",
+			Tone:      "image-generator",
+			Status:    "success",
+			LayerRole: "image-generator",
+			Width:     1024,
+			Height:    1024,
+		},
+	}
+
+	merged := mergeGeneratedNodesIntoTarget(nodes, generated, "generator-1")
+	if len(merged) != 1 {
+		t.Fatalf("expected existing target to be updated in place, got %#v", merged)
+	}
+	node := merged[0]
+	if node.Content != "https://example.com/new.png" || node.LayerRole != "" || node.Tone != "visual" {
+		t.Fatalf("expected completed image generator to become regular image, got %#v", node)
+	}
+}
+
 func TestImageGeneratorTargetNodeIDParsesDirectiveCaseInsensitively(t *testing.T) {
 	targetID := imageGeneratorTargetNodeID([]design.AgentChatMessage{
 		{
@@ -64,6 +102,36 @@ func TestImageGeneratorTargetNodeIDParsesDirectiveCaseInsensitively(t *testing.T
 	}
 }
 
+func TestRestoreFinishedGeneratedImageNodesConvertsCompletedImageGeneratorNode(t *testing.T) {
+	now := time.Now()
+	project := design.Project{
+		ID:     "project-completed-generator",
+		Status: design.StatusReady,
+		Nodes: []design.Node{
+			{
+				ID:        "generator-1",
+				Type:      design.NodeTypeImage,
+				Title:     "生成图片",
+				Content:   "http://localhost:19000/canvas-assets/cat.png",
+				Tone:      "image-generator",
+				Status:    "success",
+				LayerRole: "image-generator",
+				Width:     1024,
+				Height:    1024,
+			},
+		},
+	}
+
+	restored, changed := (*DesignService)(nil).restoreFinishedGeneratedImageNodes(project, now.Add(time.Second))
+	if !changed {
+		t.Fatal("expected completed image generator node to be normalized")
+	}
+	node := restored.Nodes[0]
+	if node.LayerRole != "" || node.Tone != "visual" || node.Status != "success" || node.Content != "http://localhost:19000/canvas-assets/cat.png" {
+		t.Fatalf("expected regular completed image node, got %#v", node)
+	}
+}
+
 func TestRestoreFinishedGeneratedImageNodesRepairsStaleCanvasSnapshot(t *testing.T) {
 	now := time.Now()
 	project := design.Project{