dsh-worktree: Git worktree isolation and delivery for DSH subagents

`dsh-worktree` is a Git worktree plugin for DSH designed to resolve write conflicts when multiple sub-agents process the same repository in parallel. By providing independent worktrees for sub-agents, it supports a complete delivery workflow including review, verification, commit, merge, push, Pull Request creation, and archive recovery. The plugin is installed within a global DSH profile and does not modify business project configurations. Users can delegate tasks via `subagent_worktree` or manually manage worktrees using the `/worktree` command. The plugin supports various worktree origins and retention modes, includes built-in safety mechanisms to prevent the accidental deletion of modified worktrees, and does not use force push. It is suitable for scenarios requiring parallel development while maintaining result traceability. Users must review the source code and license before use, and uninstalling the plugin will not automatically delete files.