Fix auto-import to set closed_at timestamp for closed issues

- Auto-import now automatically sets closed_at for closed issues missing it
- Fixes TestAutoImportClosedAtInvariant test failure
- Ensures closed_at invariant is maintained during import

Amp-Thread-ID: https://ampcode.com/threads/T-2895c896-d5ef-4639-adc8-f46d76cad451
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/main.go

@@ -861,6 +861,12 @@ func autoImportIfNewer() {
 			return
 		}
 
+		// Fix closed_at invariant: closed issues must have closed_at timestamp
+		if issue.Status == types.StatusClosed && issue.ClosedAt == nil {
+			now := time.Now()
+			issue.ClosedAt = &now
+		}
+
 		allIssues = append(allIssues, &issue)
 	}