labs ts-opencode-watchdog advertisements

On labs, /usr/local/bin/ts-opencode-watchdog.sh captures tailscale debug prefs once, independently restores svc:opencode with serve advertise, and restores missing 0.0.0.0/0 or ::/0 routes with set --advertise-exit-node. Chose extending the existing 60-second ts-opencode-advertise.timer over adding another recovery unit because one mechanism avoids overlap and preserves tags and unrelated preferences. A mutation test cleared AdvertiseRoutes; the timer restored both routes within one minute while preserving the service advertisement and clean Tailscale health.