dsh-branchman: Tree-based Branch Management Plugin for DeepSeek Harness

`dsh-branchman` is a DeepSeek Harness plugin designed to address the disconnect between session branch management and code repository branch management. It maps branch operations to Git worktrees, sub-sessions, and a visual tree structure, supporting session inheritance, file carrying, conflict handling, and interruption recovery. Developers can use the sidebar’s “Path Overview” for visual management, making it suitable for users who need to clearly manage development directions in multi-branch environments.