dsh-db-connector: Secure Database Connector for DeepSeek Harness

`db-connector` is a standalone database connection plugin for DeepSeek Harness (DSH), supporting SQLite, PostgreSQL, and MySQL. It ensures data security through named connections, lazy opening, and credential reference management. Core features include schema introspection, read-only protection (blocking all statements other than SELECT), and a write approval mechanism. All operations generate JSONL audit logs. Installation requires Node.js >= 22.16, configuration supports YAML, and it registers three tools—`db_connect`, `db_schema`, and `db_query`—to manage connections, schemas, and queries.