wilshi/dsh-external-api: Add an optional external HTTP API to DSH Desktop

The DSH Desktop plugin maintained by WilShi optionally mounts an HTTP API within the process that is disabled by default and bound to 127.0.0.1 by default. It authenticates using Bearer Tokens and exposes sessions, Run, SSE events, and capability probing. This article details installation commands, enabling steps, interfaces and Scope tables, browser fetch methods for subscribing to SSE, and error formats, as well as security boundaries such as Token isolation, plaintext HTTP, and concurrency limits. Before installation, one should read the source code, confirm the MIT license, and understand that the plugin operates with the privileges of the current DSH process.