security: add mDNS discovery config to reduce information disclosure (#1882)

* security: add mDNS discovery config to reduce information disclosure

mDNS broadcasts can expose sensitive operational details like filesystem
paths (cliPath) and SSH availability (sshPort) to anyone on the local
network. This information aids reconnaissance and should be minimized
for gateways exposed beyond trusted networks.

Changes:
- Add discovery.mdns.enabled config option to disable mDNS entirely
- Add discovery.mdns.minimal option to omit cliPath/sshPort from TXT records
- Update security docs with operational security guidance

Minimal mode still broadcasts enough for device discovery (role, gatewayPort,
transport) while omitting details that help map the host environment.
Apps that need CLI path can fetch it via the authenticated WebSocket.

* fix: default mDNS discovery mode to minimal (#1882) (thanks @orlyjamie)

---------

Co-authored-by: theonejvo <orlyjamie@users.noreply.github.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/gateway/server.impl.ts

@@ -352,6 +352,7 @@ export async function startGatewayServer(
       : undefined,
     wideAreaDiscoveryEnabled: cfgAtStart.discovery?.wideArea?.enabled === true,
     tailscaleMode,
+    mdnsMode: cfgAtStart.discovery?.mdns?.mode,
     logDiscovery,
   });
   bonjourStop = discovery.bonjourStop;