refactor(tailscale): rename exit-node to subnet-router; drop --advertise-exit-node

The host was always intended as a LAN subnet router (--advertise-routes),
not a full exit node (--advertise-exit-node). Rename every trace of
"exit-node" to "subnet-router" and remove the --advertise-exit-node flag
from extraSetFlags; the operator supplies --advertise-routes at first
tailscale up and Tailscale persists it in state across reboots.

Routing sysctls (useRoutingFeatures = "server"), openFirewall, and
trustedInterfaces = ["tailscale0"] are still required for subnet routing
to work, so the module is kept — just correctly named.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TF2dsuKZAiyZWQ1D7CuHJm

modules/tailscale/exit-node.nix

@@ -1,27 +0,0 @@
-_:
-
-{
-  imports = [ ./enable-service.nix ];
-
-  services.tailscale = {
-    # Enables the sysctl forwarding settings exit nodes/subnet routers need;
-    # without this, --advertise-exit-node has no effect.
-    useRoutingFeatures = "server";
-
-    # Lets peers reach this node directly over the tailscale UDP port
-    # instead of relaying through DERP.
-    openFirewall = true;
-
-    # extraSetFlags (tailscale set, via the always-on tailscaled-set
-    # service), not extraUpFlags -- extraUpFlags is only ever applied by
-    # tailscaled-autoconnect, which itself only runs when
-    # services.tailscale.authKeyFile is set (nothing in this repo sets one,
-    # so tailscale up is a manual, one-time operator step on every host that
-    # uses this service). extraSetFlags has no such gate, so
-    # --advertise-exit-node self-reapplies on every boot once the operator
-    # has authenticated the node once.
-    extraSetFlags = [
-      "--advertise-exit-node"
-    ];
-  };
-}

modules/tailscale/subnet-router.nix

@@ -0,0 +1,15 @@
+_:
+
+{
+  imports = [ ./enable-service.nix ];
+
+  services.tailscale = {
+    # Enables the sysctl forwarding settings subnet routers need;
+    # without this, --advertise-routes has no effect.
+    useRoutingFeatures = "server";
+
+    # Lets peers reach this node directly over the tailscale UDP port
+    # instead of relaying through DERP.
+    openFirewall = true;
+  };
+}