Remove pinned UID for mcp user

UID 995 conflicted with sshd on orion. Let NixOS auto-assign the UID
for the mcp system user. Use systemd's %U specifier for XDG_RUNTIME_DIR
instead of the hardcoded UID.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

configs/mcp.nix

@@ -4,13 +4,9 @@
 # and a systemd service for the agent daemon.
 { pkgs, ... }:
 
-let
-  mcpUid = 995;
-in
 {
   users.users.mcp = {
     isSystemUser = true;
-    uid = mcpUid; # Pin UID so systemd Environment references stay stable.
     group = "mcp";
     home = "/srv/mcp";
     shell = pkgs.shadow; # nologin equivalent
@@ -42,7 +38,7 @@ in
       Group = "mcp";
       Environment = [
         "HOME=/srv/mcp"
-        "XDG_RUNTIME_DIR=/run/user/${toString mcpUid}"
+        "XDG_RUNTIME_DIR=/run/user/%U"
         "PATH=/run/current-system/sw/bin:/usr/local/bin"
       ];