dsh-session-share: Offline Session Tree Sharing Between DeepSeek Harnesses

The `dsh-session-share` plugin for DeepSeek Harness (DSH) allows users to export complex Sessions, including multi-level nested sub-agent trees, images, and tool records, into local ZIP capsules for offline transfer between instances. The plugin supports downloading the capsule via `/share-session` and importing it via `/import-session`, with each import generating a new local Session tree. Since v1 lacks encryption, it is recommended to use it in a controlled, trusted environment to ensure secure transfer.