dsh-rename: Add a `/rename` command to rename sessions in DeepSeek Harness

`dsh-rename` is an MIT-licensed DeepSeek Harness (DSH) plugin developed by lmst2, designed to supplement the limitations of built-in session auto-naming and provide flexible session renaming functionality. The plugin supports two modes via the `/rename` slash command: if a specific name is provided, it renames the session directly; if no name is provided, it allows the model to read the current context and generate a concise title. The renamed title is fixed to prevent it from being overwritten by subsequent auto-naming. The plugin relies on DSH's built-in event submission mechanism and does not write custom session data, ensuring that historical sessions remain compatible with any DSH build even after uninstallation. It supports customizing title length, input/output budget, timeout, and model routing in profile configuration, but requires provider and m...