fix(#2305): run_cmd returns Result, nix fmt, collapse nested if

- run_cmd now returns Result<String, String> — callers pattern-match
  instead of comparing against an "ok" sentinel string
- Add cmd_result() helper to format run_cmd results as tool strings
- mouse_click: collapse nested if-let into let-chain (clippy collapsible_if)
- nix fmt: reformat screen.nix package list

nix/agent-modules/screen.nix

@@ -40,9 +40,11 @@
 
     # grim: Wayland screenshot; wtype: text/key input (no daemon).
     # ydotool: mouse + key injection via uinput (needs screenInput).
-    environment.systemPackages =
-      [ pkgs.grim pkgs.wtype ]
-      ++ lib.optional config.hyperhive.gui.screenInput pkgs.ydotool;
+    environment.systemPackages = [
+      pkgs.grim
+      pkgs.wtype
+    ]
+    ++ lib.optional config.hyperhive.gui.screenInput pkgs.ydotool;
 
     # ydotoold — uinput event injection daemon. The socket lands at
     # /tmp/.ydotool_socket by default; ydotool picks it up