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/migrations.go

@@ -45,6 +45,7 @@ var migrationsList = []Migration{
 	{"additional_indexes", migrations.MigrateAdditionalIndexes},
 	{"gate_columns", migrations.MigrateGateColumns},
 	{"tombstone_closed_at", migrations.MigrateTombstoneClosedAt},
+	{"created_by_column", migrations.MigrateCreatedByColumn},
 }
 
 // MigrationInfo contains metadata about a migration for inspection