flake: add git-sync-rs as top-level input and deduplicate

Move git-sync-rs from a nixpkgs PR patch to a direct flake input,
add overlay and package, and have org-agenda-api follow it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nixos/essential.nix

@@ -25,6 +25,7 @@
     git-fame
     git-lfs
     git-sync
+    git-sync-rs
     gitFull
     gnumake
     gparted