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/029_created_by_column.go

@@ -0,0 +1,34 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// MigrateCreatedByColumn adds the created_by column to the issues table.
+// This tracks who created the issue, using the same actor chain as comment authors
+// (--actor flag, BD_ACTOR env, or $USER). GH#748.
+func MigrateCreatedByColumn(db *sql.DB) error {
+	// Check if column already exists
+	var columnExists bool
+	err := db.QueryRow(`
+		SELECT COUNT(*) > 0
+		FROM pragma_table_info('issues')
+		WHERE name = 'created_by'
+	`).Scan(&columnExists)
+	if err != nil {
+		return fmt.Errorf("failed to check created_by column: %w", err)
+	}
+
+	if columnExists {
+		return nil
+	}
+
+	// Add the created_by column
+	_, err = db.Exec(`ALTER TABLE issues ADD COLUMN created_by TEXT DEFAULT ''`)
+	if err != nil {
+		return fmt.Errorf("failed to add created_by column: %w", err)
+	}
+
+	return nil
+}