dsh-session-persistence-mysql: MySQL Persistence Backend for DSH

The `dsh-session-persistence-mysql` plugin provides a MySQL-based session persistence solution for DeepSeek Harness. It implements the `SessionPersistence` interface and supports transactional appending, tear-free tail handling, crash recovery, and read/write separation. The plugin is configured through environment variables and supports lazy instantiation and a deadlock retry mechanism. Known limitations include no support for deletion or paginated filtering, and the use of plaintext storage. It is suitable for production environments with high data reliability requirements.