dsh-thread: Session Memory Plugin for DeepSeek Harness

The DeepSeek Harness (DSH) plugin `dsh-thread` provides a deep conversational memory solution, using an embedded SQLite database to store event streams and ensuring that coding agents maintain contextual coherence over long-duration interactions. It delivers core capabilities such as lossless capture, structured state injection, deterministic BM25-based retrieval, behavior-contract skills, and explicit decision/preference logging. The plugin supports session isolation, resource cleanup, and wrap-up deposition, helping agents overcome memory fragmentation and achieve accurate, coherent long-running operation.