labs exit-node advertisement

Always keep the Tailscale machine labs advertised as an exit node and keep svc:opencode advertised. Chose unified automatic recovery [[019eefd0-fb83-7d6b-ade4-0c453ca1061e]] over manual re-advertisement because re-authentication and tailscale up can clear either preference; the enabled timer restores both within about one minute without disturbing tags or unrelated settings.