Add Nix development shell to regen api.proto.

To use, run `nix develop` to enter a nested shell with protoc, run
regen.sh, and then exit the shell.

Requires Nix's flake features to be enabled.

Files have been regenerated with a slightly more recent protobuffers
version, but no functional change.

flake.nix

@@ -0,0 +1,27 @@
+{
+  inputs = {
+    nixpkgs.url = "nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs, ... }@inputs:
+    let
+      supportedSystems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forAllSystems = fn:
+        nixpkgs.lib.genAttrs supportedSystems
+        (system: fn { pkgs = import nixpkgs { inherit system; }; });
+    in {
+      devShells = forAllSystems ({ pkgs }: {
+        default = pkgs.mkShell {
+          nativeBuildInputs = with pkgs; [
+            go_1_23
+            protobuf
+          ];
+        };
+      });
+    };
+}