fix: add nix and git to PATH for flake collector

The systemd service runs in a sandboxed environment without the usual
PATH. Add nix and git to the service path when flake collector is
enabled so it can run `nix flake metadata` on git+https:// URLs.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

module.nix

@@ -85,6 +85,12 @@ in
       wantedBy = [ "multi-user.target" ];
       after = [ "network.target" ];
 
+      # nix is required for flake collector, git for git+https:// URLs
+      path = lib.mkIf cfg.flake.enable [
+        config.nix.package
+        pkgs.git
+      ];
+
       serviceConfig = {
         User = cfg.user;
         Group = cfg.group;