fexp-file-explorer: DSH Left-Side Workspace File Browser

`fexp-file-explorer` is a dynamic Cordis plugin for DeepSeek Harness (DSH), designed to integrate file browsing capabilities into the left-side workspace. The plugin provides a standalone 320px file management panel accessible via a sidebar capsule button or title-bar entry, supporting directory navigation, breadcrumb jumps, and file content preview (text limited to 1 MB). Users can add file references to chat using the `@` command or open a directory in the system file explorer. This plugin is packaged as a static bundle, automatically loads with a profile, and has a WARN security review level.