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_reliability_test.go

@@ -171,7 +171,11 @@ func TestCloudAgentReliabilityFailedContinuation(t *testing.T) {
 		t.Fatal(err)
 	}
 	t.Logf("failed first turn -> continue: inherited history messages=%d", len(input.TextHistory))
-	if len(input.TextHistory) != 2 || input.TextHistory[0].Content != agentTestRequest().Prompt || !strings.Contains(input.TextHistory[1].Content, "failed") {
+	if len(input.TextHistory) != 3 ||
+		input.TextHistory[0].Content != agentTestRequest().Prompt ||
+		input.TextHistory[1].Role != "assistant" ||
+		input.TextHistory[2].Role != "user" ||
+		!strings.Contains(input.TextHistory[2].Content, "failed") {
 		t.Fatalf("continuation lost facts: %+v", input.TextHistory)
 	}
 }