dsh-daemon: Register dsh-web as an auto-start, self-healing background service

dsh-daemon is a DSH plugin designed to register `dsh web` as a self-starting, self-healing user-level background service, resolving process instability and difficulty in checking status caused by manual foreground startup. The plugin supports macOS (LaunchAgent), Linux (systemd or cron), and Windows (Scheduled Tasks, not verified on real machines) platforms. Its core watchdog script performs a health check every 30 seconds, automatically restarting the service after three consecutive failures or waking from sleep. The plugin provides 7 agent tools and the `dsh-daemon` CLI for installation, status queries, start/stop, and update management. It supports automatic updates for the same major version, with selectable modes of "restart" or "download and apply"; manual execution is required for major version changes.