dsh-logbook: A DSH Plugin That Makes ctx.logger Output Visible

`dsh-logbook` is a DeepSeek Harness plugin designed to address the issue of invisible default logs. It provides a `/logs` command and a stderr exporter to make the output of `ctx.logger` visible. It supports filtering logs by level and name, and allows flexible configuration of log retention counts and muting specific loggers in `cordis.patch.yml`. This plugin resolves the bug where buffered warn/debug logs are silently discarded, making it a practical tool for troubleshooting in the terminal.