fix(tailscale): rename hosts/tailscale-subnet-router → hosts/tailscale-router

The directory name was tailscale-subnet-router but networking.hostName
was already tailscale-router, causing a mismatch that confused scripts
comparing directory paths against actual hostnames. Flake attribute
names (linode/proxmox/lxc-tailscale-subnet-router) and .sops.yaml
anchors are unchanged — they describe the build type, not the hostname.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hosts/tailscale-router/host.nix

@@ -0,0 +1,10 @@
+_:
+
+{
+  networking.hostName = "tailscale-router";
+
+  # No networking.hostId: only ZFS-touching hosts (server, docker) need one
+  # for pool-import safety, and this host does neither.
+
+  system.stateVersion = "26.05";
+}