dsh-pr-watch: DeepSeek Harness Plugin

The DeepSeek Harness agent lacks PR tracking memory, causing duplicate API calls. The `dsh-pr-watch` plugin resolves this by storing snapshots and reporting incremental changes. It supports cross-repository tracking, status differentiation (merged/closed/stale), and duplicate-report prevention. It relies solely on GitHub CLI authentication and does not require local repository cloning. Currently in pre-release, it must be built and installed from source and requires Node.js 18 or later.