Update flake.lock and fix synergy build with GCC 15

- Update flake inputs (home-manager, nix, nixos-wsl, nixpkgs, org-agenda-api)
- Add overlay to fix synergy missing #include <cstdint>

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

nixos/overlay.nix

@@ -162,6 +162,13 @@ in
   in
   final.python3.withPackages my-python-packages;
 
+  # Fix synergy build with GCC 15 - missing #include <cstdint>
+  synergy = prev.synergy.overrideAttrs (oldAttrs: {
+    postPatch = (oldAttrs.postPatch or "") + ''
+      sed -i '/#include <cstring>/a #include <cstdint>' src/lib/server/InputFilter.cpp
+    '';
+  });
+
   pythonPackagesExtensions = prev.pythonPackagesExtensions ++ [
     (
       python-final: python-prev: {