dsh-mysql: Add MySQL connection and database control tools for DSH

dsh-mysql is a Web profile plugin for DeepSeek Harness (DSH) designed to address scattered MySQL connection configurations, accidental session selection, and unclear permission boundaries. Adopting the MIT license, this plugin allows for the global registration of tools without the need to modify Agent presets. Its core features include a settings page for managing multiple MySQL connection sets, supporting the addition, editing, deletion, and testing of connections; a session-based database selector located beside the input box to ensure connection selections between sessions do not interfere with each other; and three controlled tools: `mysql_tables` for discovering table structures, `mysql_query` for executing queries (limited to 2000 rows), and `mysql_execute` for controlled write operations. The plugin is read-only by default, and write permissions must be explicitly enabled for each connection.