dsh-self-update: In-app self-update plugin for DeepSeek Harness installed via Git source

`dsh-self-update` is a self-update plugin tailored for DeepSeek Harness (DSH) git checkout installation scenarios. It streamlines the tedious manual processes of executing `git pull`, installing dependencies, and building/restarting, while providing in-app one-click updates, failure rollback, and service restart capabilities. The plugin checks for new versions in the background every 6 hours or on demand, displaying an update panel via a sidebar entry. The update process automatically executes standard build commands and refuses to modify dirty working trees to preserve local changes; if a branch divergence exists, it backs up local commits to a separate branch before synchronizing with the remote. It supports one-click rollback and CLI fallback commands in case of update failures. Service restart is implemented via process exit code 75 and is compatible with systemd, PM2, and the native macOS Shell (requires Xcode toolchain; the application uses Ad-hoc signing).