dsh-workflow-worktree: Git Worktree Isolation Backend for DeepSeek Harness Workflows

`dsh-workflow-worktree` is an isolation backend plugin for DeepSeek Harness, designed to address the issue that the `@dsh-external/workflow` plugin does not implement isolation logic. It provides each agent with an isolated Git worktree, ensuring that changes are preserved after task completion without polluting the main repository. The plugin supports crash survival, maintains fingerprint cleanliness, and refuses to run in invalid directories. It records state through a manifest and uses `.git/info/exclude` to keep the main repository clean. The main limitations are that it supports single repositories only and does not handle merge logic.