Add comprehensive unit tests for CreateIssues (bd-241)

- Added 12 table-driven test cases covering all validation scenarios
- Added 2 rollback tests (validation error + DB conflict)
- Added nil item detection to prevent panic
- Fixed nil pointer panic in CreateIssues
- Tests verify ID uniqueness, timestamps, closed_at invariant
- All tests pass

Amp-Thread-ID: https://ampcode.com/threads/T-61c584cd-d873-4a1a-bfa6-d739b630b3e5
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/sqlite.go

@@ -518,9 +518,13 @@ func (s *SQLiteStorage) CreateIssues(ctx context.Context, issues []*types.Issue,
 		return nil
 	}
 
-	// Phase 1: Set timestamps and validate all issues first (fail-fast)
+	// Phase 1: Check for nil and validate all issues first (fail-fast)
 	now := time.Now()
 	for i, issue := range issues {
+		if issue == nil {
+			return fmt.Errorf("issue %d is nil", i)
+		}
+
 		issue.CreatedAt = now
 		issue.UpdatedAt = now