Close trial task cards and keep long histories visible

Why: pipefail treated a recent merged-card match as a failed git log after grep exited early, hiding completed cards from precheck and task-close.

Evidence: A disposable long-history regression failed before the fix and passed after it; make test-governance passed 341 cases. Git history confirms PR #489/#488/#486 merge SHAs.

Impact: Detect merged cards reliably and archive only the three authorized trial cards; four unrelated eligible cards remain untouched.

Refs: Dev merge queue #518; Dev #489, #488, #486

tests/test_task.py

@@ -758,6 +758,35 @@ class TaskLifecycleTests(unittest.TestCase):
         self.git(publisher, "push", "origin", "main")
         return self.git(publisher, "rev-parse", "HEAD").stdout.strip(), card.read_text()
 
+    def test_long_merge_history_keeps_merged_card_visible_and_closable(self):
+        # A recent match makes grep -q/head exit before git log has finished
+        # writing a large history. With pipefail, its SIGPIPE used to hide the
+        # card from precheck and leave task-close with nothing to archive.
+        base = self.git(self.repo, "rev-parse", "HEAD").stdout.strip()
+        tree = self.git(self.repo, "rev-parse", "HEAD^{tree}").stdout.strip()
+        side = self.git(self.repo, "commit-tree", tree, "-p", base,
+                        "-m", "Fixture side commit").stdout.strip()
+        tip = base
+        for i in range(220):
+            tip = self.git(self.repo, "commit-tree", tree, "-p", tip, "-p", side,
+                           "-m", f"Older fixture merge {i} " + "x" * 512).stdout.strip()
+        self.git(self.repo, "update-ref", "refs/heads/main", tip)
+        self.git(self.repo, "push", "origin", "main")
+        actual_merge, _ = self.publish_merged_task()
+        self.git(self.repo, "fetch", "origin")
+        self.git(self.repo, "merge", "--ff-only", "origin/main")
+
+        precheck = self.task(self.repo, "precheck")
+        self.assertIn("[cards] 已合入未闭合任务卡:feature-merged.md", precheck.stdout)
+        closed = self.task(self.repo, "close")
+        self.assertIn("[closed] 1 张已合入卡片", closed.stdout)
+        refs = self.git(self.repo, "for-each-ref", "--format=%(refname:short)",
+                        "refs/heads/feature/card-close-*").stdout.splitlines()
+        self.assertEqual(len(refs), 1)
+        archived = self.git(self.repo, "show", f"{refs[0]}:.task/archive/feature-merged.md").stdout
+        self.assertIn("状态：已完成（回填）", archived)
+        self.assertIn(actual_merge, archived)
+
     def assert_post_merge_only_reminds_owner(self, operation):
         expected_head, expected_card = self.publish_merged_task()
         self.git(self.repo, "fetch", "origin")