dsh-better-at: Move DSH Web's @ mention index to the browser

# Introduction In the DeepSeek Harness (DSH) web interface, the `@` symbol serves as the primary entry point for referencing files and sessions. Under the default implementation, every keystroke triggers a request to the Host for candidate filtering; consequently, delays in menu opening and filtering accumulate significantly when there are numerous workspace files and historical sessions. dsh-better-at addresses this by pre-fetching file and session indices into the browser, performing all filtering and sorting locally, while preserving the native behavior of `@` references and mention formats. The following introduces this plugin. # What is dsh-better-at dsh-better-at is a plugin for the DSH Web GUI, positioned as a "local caching accelerator for `@` file/session references."