Implement CreateIssues batch API (bd-240, bd-244)

- Add CreateIssues method to Storage interface
- Implement SQLiteStorage.CreateIssues with atomic ID range reservation
- Single transaction for N issues (5-10x speedup expected)
- Set timestamps before validation to match CreateIssue behavior
- All tests passing

internal/storage/storage.go

@@ -11,6 +11,7 @@ import (
 type Storage interface {
 	// Issues
 	CreateIssue(ctx context.Context, issue *types.Issue, actor string) error
+	CreateIssues(ctx context.Context, issues []*types.Issue, actor string) error
 	GetIssue(ctx context.Context, id string) (*types.Issue, error)
 	UpdateIssue(ctx context.Context, id string, updates map[string]interface{}, actor string) error
 	CloseIssue(ctx context.Context, id string, reason string, actor string) error