fix(import): preserve completion timestamps on beads update

Avoid clearing an existing completed_at when a Beads closed issue is re-imported without closed_at during --update.

Co-Authored-By: gpt-5.3-codex <noreply@anthropic.com>

Author: gkze

src/cli/import.test.ts

@@ -1429,6 +1429,42 @@ Old subtask context.
       expect(child?.blockedBy).toEqual([]);
     });
 
+    it("preserves completed_at on update when Beads closed issue has no closed_at", async () => {
+      const filePath = writeBeadsFixture(
+        `${JSON.stringify({
+          id: "completed-preserve",
+          title: "Completed v1",
+          status: "closed",
+          priority: 1,
+          closed_at: "2026-01-05T00:00:00Z",
+        })}\n`,
+      );
+
+      await runCli(["import", "--beads", filePath], { storage });
+
+      writeBeadsFixture(
+        `${JSON.stringify({
+          id: "completed-preserve",
+          title: "Completed v2",
+          status: "closed",
+          priority: 1,
+        })}\n`,
+      );
+
+      await runCli(["import", "--beads", filePath, "--update"], {
+        storage,
+      });
+
+      const tasks = await storage.readAsync();
+      const task = tasks.tasks.find((item) => item.id === "completed-preserve");
+
+      expect(task?.completed).toBe(true);
+      expect(task?.name).toBe("Completed v2");
+      expect(new Date(task?.completed_at ?? "").toISOString()).toBe(
+        "2026-01-05T00:00:00.000Z",
+      );
+    });
+
     it("reports warnings for missing relationship targets without failing import", async () => {
       const filePath = writeBeadsFixture(
         `${JSON.stringify({

src/cli/import.ts

@@ -281,7 +281,11 @@ async function importFromBeadsFile(
         description: issue.description,
         priority: issue.priority,
         completed: issue.completed,
-        completed_at: issue.completed ? (issue.completed_at ?? null) : null,
+        ...(!issue.completed
+          ? { completed_at: null }
+          : issue.completed_at
+            ? { completed_at: issue.completed_at }
+            : {}),
         result: issue.completed ? issue.result : null,
         started_at: issue.started_at ?? null,
         metadata: {