dsh-ctxopt-compaction: DeepSeek Harness Context Compression Plugin

`dsh-ctxopt-compaction` is a context compaction plugin for DeepSeek Harness, designed to resolve long conversation overflow issues. It offers benefit evaluation, summary caching, explicit memory control, and diagnostic information retention, and requires host-side dependency support. Its configuration enables fine-grained control over the compaction window, cache size, and tool output trimming strategy, while allowing users to manage memories and to-do items via commands.