internal/workflow：directLastActivity 排除跨 issue 引用事件（#34）

同步 agents #34/#38/#40 三個停滯判定修補到 Go 端：

- Source 介面新增 IssueTimeline（GET /repos/{o}/{r}/issues/{n}/timeline，分頁），
  giteaapi.go 與 cmd/xcheck 同步實作
- 新增 directLastActivity：updated_at 晚於全部已知直接活動時拉 timeline 核對，
  排除 comment_ref／issue_ref 引用事件後取最晚；全為引用事件回退已知直接活動
  最晚時刻（agents #40）；timeline 失敗保守沿用 updated_at
- considerIssue／considerPull 對自身 updated_at、childActivity 對子項 updated_at
  （含子項 created_at 入已知活動）套用同一排除邏輯（agents #38）
- 回歸測試：子項 updated_at 被引用刷新（TestRunStalledChildXrefRefresh）、
  timeline 全引用事件回退與拉取失敗保守路徑（TestRunStalledTimelineAllXrefFallback）
- 修正 workflow_test.go：timelines 須在 ScanStalled 呼叫前設定

驗證：go build／vet／test 全綠；xcheck 對真實 API 掃描，
stalled 輸出與 gitea.py（agents @ 78769ec）位元組一致（5833 bytes）。

internal/cli/stalled_commands_test.go

@@ -46,6 +46,9 @@ func (s *stubStalledSource) TryGetIssue(owner, repo string, number int64) (workf
 	i, ok := s.issues[owner+"/"+repo][number]
 	return i, ok, nil
 }
+func (s *stubStalledSource) IssueTimeline(owner, repo string, number int64) ([]workflow.TimelineEvent, error) {
+	return nil, nil
+}
 func (s *stubStalledSource) GetIssue(owner, repo string, number int64) (workflow.Issue, error) {
 	if s.getErr != nil {
 		return workflow.Issue{}, s.getErr
@@ -157,7 +160,7 @@ func TestWiringXrefs(t *testing.T) {
 	src := newStubStalledData()
 	src.issues["alterminal/agents"][1] = workflow.Issue{
 		Number: 1, Title: "父項", State: "open",
-		Body: "子項：agents#2",
+		Body:       "子項：agents#2",
 		Repository: &workflow.Repository{FullName: "alterminal/agents"},
 	}
 	src.issues["alterminal/agents"][2] = workflow.Issue{
@@ -185,7 +188,7 @@ func TestWiringXrefsEmpty(t *testing.T) {
 	src := newStubStalledData()
 	src.issues["alterminal/agents"][1] = workflow.Issue{
 		Number: 1, Title: "無引用", State: "open",
-		Body: "沒有任何引用",
+		Body:       "沒有任何引用",
 		Repository: &workflow.Repository{FullName: "alterminal/agents"},
 	}
 	newStubStalledEnv(t, src, time.Date(2026, 9, 10, 8, 0, 0, 0, time.UTC))