feat(services): opt-in Tailscale + Syncthing (nomarchy.services.*)

Start the opt-in services surface: nomarchy.services.{tailscale,syncthing},
off by default, in modules/nixos/services.nix. Tailscale ships the daemon
(authenticate with `sudo tailscale up`); Syncthing runs as the login user
with its GUI on 127.0.0.1:8384 (overrideDevices/Folders off so GUI-managed
folders survive rebuilds). username is read lazily, only when Syncthing is
on. Commented examples in the downstream system.nix template per the opt-in
convention.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nixos/default.nix

@@ -22,7 +22,7 @@ let
   '';
 in
 {
-  imports = [ ./options.nix ./plymouth.nix ./file-manager.nix ./power.nix ];
+  imports = [ ./options.nix ./plymouth.nix ./file-manager.nix ./power.nix ./services.nix ];
 
   config = {
     # The safe half of distro branding: distroName flows into