feat: Add created_by field to issues (GH#748)

Add a created_by field to track who created each issue, similar to how
comments have an author field.

- Add CreatedBy string field to Issue struct
- Add migration 029 to add created_by column to issues table
- Update all SELECT/INSERT/Scan statements across storage layer
- Populate created_by in bd create from actor chain
- Display created_by in bd show output

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

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

internal/storage/sqlite/labels.go

@@ -157,7 +157,7 @@ func (s *SQLiteStorage) GetIssuesByLabel(ctx context.Context, label string) ([]*
 	rows, err := s.db.QueryContext(ctx, `
 		SELECT i.id, i.content_hash, i.title, i.description, i.design, i.acceptance_criteria, i.notes,
 		       i.status, i.priority, i.issue_type, i.assignee, i.estimated_minutes,
-		       i.created_at, i.updated_at, i.closed_at, i.external_ref, i.source_repo, i.close_reason,
+		       i.created_at, i.created_by, i.updated_at, i.closed_at, i.external_ref, i.source_repo, i.close_reason,
 		       i.deleted_at, i.deleted_by, i.delete_reason, i.original_type,
 		       i.sender, i.ephemeral, i.pinned, i.is_template,
 		       i.await_type, i.await_id, i.timeout_ns, i.waiters