dsh-notify-xc: DSH Browser Notification Plugin

`dsh-notify-xc` is a browser extension for DeepSeek Harness (DSH) that sends system-level notifications when background session tasks complete, fail, or require human intervention. It is a compatibility-focused rewrite based on `dsh-agent-notify`, tailored for DSH 0.1.2-alpha.4. Running entirely client-side in the browser, it supports three notification types—task completion, runtime errors, and waiting for input—and includes false-positive prevention, direct session access on click, workspace attribution display, and one-click clearing. Once installed, it helps developers stay on top of Agent status in real time and improve development efficiency.