Wire up service.Service in HTTP handlers and delegate cleanupUser to
svc.BroadcastQuit for consistent quit/part logic across transports.
Default IRC_LISTEN_ADDR to :6667, remove unused import, fix all lint
issues (dogsled, funcorder, wrapcheck, varnamelen, nolintlint).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
