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/nix.nix

@@ -61,6 +61,7 @@
       (final: prev: {
         codex = inputs.codex-cli-nix.packages.${prev.stdenv.hostPlatform.system}.default;
         claude-code = inputs.claude-code-nix.packages.${prev.stdenv.hostPlatform.system}.default;
+        git-sync-rs = inputs.git-sync-rs.packages.${prev.stdenv.hostPlatform.system}.default;
       })
     ] ++ (if config.imalison.nixOverlay.enable then [ inputs.nix.overlays.default ] else []);