dsh-tui-checkpoints: dsh-TUI Workspace Shadow Snapshots

dsh-tui-checkpoints is a community plugin for dsh-TUI, developed to address the challenge of rolling back file states in agent development. Through a shadow repository mechanism, it automatically saves workspace states at conversation turn boundaries. Its core features include automatic snapshots, a dedicated file rollback panel, safe restore mechanisms with undo support, and nested repository handling. Users can browse snapshot diffs with the `/checkpoints` command and restore file states as needed. Combined with dsh-TUI's built-in conversation rollback feature, it helps maintain consistency between file states and conversations. It is currently in the Spike phase, requires installation from source, and carries the risk that unsaved edits may be overwritten.