Fix deprecated NixOS options to resolve flake check warnings

- Update system attribute to stdenv.hostPlatform.system in overlays
- Migrate git config to new settings structure (user.name, user.email)
- Move ssh.addKeysToAgent to matchBlocks configuration
- Disable ssh default config to prevent future deprecation warnings

flake.nix

@@ -53,7 +53,7 @@
         nixpkgs.overlays = [
           (final: prev: {
             unstable = import nixpkgs-unstable {
-              system = prev.system;
+              system = prev.stdenv.hostPlatform.system;
               config.allowUnfree = true;
             };
             custom = prev.callPackage ./packages {};
@@ -81,7 +81,7 @@
         nixpkgs.overlays = [
           (final: prev: {
             unstable = import nixpkgs-unstable {
-              system = prev.system;
+              system = prev.stdenv.hostPlatform.system;
               config.allowUnfree = true;
             };
             custom = prev.callPackage ./packages {};
@@ -107,7 +107,7 @@
         nixpkgs.overlays = [
           (final: prev: {
             unstable = import nixpkgs-unstable {
-              system = prev.system;
+              system = prev.stdenv.hostPlatform.system;
               config.allowUnfree = true;
               overlays = [
                 # Override claude-code in unstable to use our custom GCS-based build