Add PostgreSQL repository implementation

[dahlia]

Similar to the existing @fedify/botkit-sqlite package, we should create a @fedify/botkit-postgres package that implements the Repository interface using PostgreSQL as the backend storage.

This would involve creating a new workspace package packages/botkit-postgres/ that implements the Repository interface using PostgreSQL, supports connection pooling and configuration options, includes proper TypeScript types, has comprehensive tests and documentation with usage examples, and supports both Deno and Node.js environments following existing patterns.

The implementation should follow the same patterns as @fedify/botkit-sqlite, use a well-established PostgreSQL driver, consider using prepared statements for performance, include proper migration/schema setup utilities, and handle connection errors gracefully. The package should build successfully for both Deno and Node.js, have all Repository interface methods implemented, pass tests in both runtime environments, and be ready for publishing to JSR and npm.