dsh-task-worktree: Isolated git worktree checkouts for each task

# Introduction A common issue when letting agents work in a repository is that all changes end up in the same workspace: changes from the previous task haven't been committed yet while the next task has started, mixing uncommitted content together. If you try to use branch switching for isolation, you must first deal with the current state. Git's native worktree allows maintaining multiple checkouts of the same repository simultaneously, each with its own attached branch. `dsh-task-worktree` integrates this capability into the DeepSeek Harness (hereinafter referred to as DSH) session model: it provides one isolated checkout per task, keeps the main workspace static, and leaves handover and cleanup actions to be performed by humans. Below, we introduce its positioning, features, and usage. # What is this `dsh-task-worktree` is Letter202