feat(agent): Agent 工作台 - 计划插话记忆、统一模型选择器并修复摘要与视频入库

将计划清单、运行中插话和个人记忆接入云端 Agent；全站模型选择器统一为左右二级菜单。画布摘要按预算截断不再整轮拒绝，首页视频未知宽高可入库。README 补充贡献者 morrow。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/app/cloud_agent_empty_output_test.go

@@ -0,0 +1,57 @@
+package app
+
+import (
+	"testing"
+
+	"infinite-canvas/backend/internal/model"
+)
+
+func TestCloudAgentEmptyModelOutputDetection(t *testing.T) {
+	cases := []struct {
+		name string
+		task *model.Task
+		want bool
+	}{
+		{"空回合 → 可恢复", &model.Task{Status: model.TaskStatusFailed, Error: "画布 Agent 接口没有返回内容"}, true},
+		{"成功的任务不算空回合", &model.Task{Status: model.TaskStatusSucceeded, Error: ""}, false},
+		{"别的失败原因不该被吞", &model.Task{Status: model.TaskStatusFailed, Error: "上游 429 限流"}, false},
+		{"nil 安全", nil, false},
+	}
+	for _, c := range cases {
+		if got := cloudAgentEmptyModelOutput(c.task); got != c.want {
+			t.Fatalf("%s：cloudAgentEmptyModelOutput = %v, want %v", c.name, got, c.want)
+		}
+	}
+	if cloudAgentMaxEmptyOutputNudges <= 0 || cloudAgentMaxEmptyOutputNudges > 3 {
+		t.Fatalf("空回合催促上限应在 1..3 之间，got %d", cloudAgentMaxEmptyOutputNudges)
+	}
+}
+
+func TestCloudAgentTruncatedToolArguments(t *testing.T) {
+	if !cloudAgentTruncatedToolArguments(&model.Task{Error: "工具参数不是完整 JSON"}) {
+		t.Fatal("截断的工具参数应判为可恢复")
+	}
+	if cloudAgentTruncatedToolArguments(&model.Task{Error: "上游 429 限流"}) {
+		t.Fatal("其它失败原因不该被当成截断")
+	}
+}
+
+func TestCloudAgentOutputViolation(t *testing.T) {
+	if got := cloudAgentOutputViolation(stringsRepeat("x", 32001), 1); got == "" {
+		t.Fatal("超长正文应判定违规")
+	}
+	if got := cloudAgentOutputViolation("ok", 9); got == "" {
+		t.Fatal("超过 8 个工具调用应判定违规")
+	}
+	if got := cloudAgentOutputViolation("ok", 2); got != "" {
+		t.Fatalf("合规输出不应违规：%s", got)
+	}
+}
+
+func stringsRepeat(value string, count int) string {
+	out := make([]byte, 0, len(value)*count)
+	for i := 0; i < count; i++ {
+		out = append(out, value...)
+	}
+	return string(out)
+}