fix(import): defensive handling for closed issues missing closed_at timestamp

Fixes GH#523: older versions of bd could close issues without setting
the closed_at timestamp. When importing such issues, validation would
fail with "closed issues must have closed_at timestamp".

This fix adds defensive handling in all issue creation/validation paths:
- If status is "closed" and closed_at is nil, set closed_at to
  max(created_at, updated_at) + 1 second
- Similarly for tombstones missing deleted_at

Applied to:
- batch_ops.go: validateBatchIssuesWithCustomStatuses (main import path)
- transaction.go: CreateIssue and CreateIssues
- queries.go: CreateIssue
- multirepo.go: upsertIssueInTx

Also adds comprehensive tests for the defensive fix.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/queries.go

@@ -50,16 +50,41 @@ func (s *SQLiteStorage) CreateIssue(ctx context.Context, issue *types.Issue, act
 		return fmt.Errorf("failed to get custom statuses: %w", err)
 	}
 
+	// Set timestamps first so defensive fixes can use them
+	now := time.Now()
+	if issue.CreatedAt.IsZero() {
+		issue.CreatedAt = now
+	}
+	if issue.UpdatedAt.IsZero() {
+		issue.UpdatedAt = now
+	}
+
+	// Defensive fix for closed_at invariant (GH#523): older versions of bd could
+	// close issues without setting closed_at. Fix by using max(created_at, updated_at) + 1s.
+	if issue.Status == types.StatusClosed && issue.ClosedAt == nil {
+		maxTime := issue.CreatedAt
+		if issue.UpdatedAt.After(maxTime) {
+			maxTime = issue.UpdatedAt
+		}
+		closedAt := maxTime.Add(time.Second)
+		issue.ClosedAt = &closedAt
+	}
+
+	// Defensive fix for deleted_at invariant: tombstones must have deleted_at
+	if issue.Status == types.StatusTombstone && issue.DeletedAt == nil {
+		maxTime := issue.CreatedAt
+		if issue.UpdatedAt.After(maxTime) {
+			maxTime = issue.UpdatedAt
+		}
+		deletedAt := maxTime.Add(time.Second)
+		issue.DeletedAt = &deletedAt
+	}
+
 	// Validate issue before creating (with custom status support)
 	if err := issue.ValidateWithCustomStatuses(customStatuses); err != nil {
 		return fmt.Errorf("validation failed: %w", err)
 	}
 
-	// Set timestamps
-	now := time.Now()
-	issue.CreatedAt = now
-	issue.UpdatedAt = now
-
 	// Compute content hash (bd-95)
 	if issue.ContentHash == "" {
 		issue.ContentHash = issue.ComputeContentHash()