dsh-readcache: Version Token Validation Cache for DSH read Tool

`dsh-readcache` is an in-process result cache plugin for the DSH read tool, designed to address the I/O and rendering overhead caused by agents repeatedly re-reading unchanged files during long sessions. Its core features include using version tokens (file metadata) validation to ensure cache consistency, supporting agent isolation to prevent race conditions, and managing memory with an LRU policy. It intercepts the event waterfall and returns a cache hit only when the file is unchanged and the secondary stat validation passes, significantly improving performance. After installation, it can be monitored and managed with the `stats` and `clear` commands.