Track .envrc + make postgres scripted test work on NixOS (#5)

* Track .envrc so devshell auto-loads on cd

Track the .envrc with `use flake` so contributors with direnv +
nix-direnv get the devshell automatically on cd, without having to
author .envrc themselves.

Closes #4

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Make postgres scripted test work on NixOS

zonky's bundled generic-Linux Postgres binaries can't run on NixOS, so
`sbt scripted` fails locally even though CI (Ubuntu) is fine.

The flake devshell now exports NOMAD_PG_TARBALL pointing at an XZ tar
of pkgs.postgresql (bin, lib, share). When set, the test feeds that
through a custom PgBinaryResolver and overrides unix_socket_directories
to $TMPDIR (nix postgres defaults to /run/postgresql, absent in the
sandbox). Without the env var, behaviour is unchanged — CI keeps
working as-is.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Fix misleading unix_socket_directories comment

The comment said "point it at the data dir" but the code uses the JVM
temp dir.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Add Darwin systems to flake devShell outputs

Extends supportedSystems with x86_64-darwin and aarch64-darwin so macOS
contributors using nix-darwin can enter the same direnv/flake devshell.

The NOMAD_PG_TARBALL override path in the postgres scripted test is
platform-agnostic, so the same Nix-built tarball workaround works on
Darwin too (harmless there since zonky's bundled macOS binaries would
also load natively).

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Ángel Felipe Blanco Guzmán <novamage@magaran.com>

Author: 3 people

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -51,5 +51,4 @@ metals.sbt
 # Claude Code
 .claude/
 MEMORY.md
-.envrc
 .direnv/

flake.nix

@@ -5,19 +5,29 @@
 
   outputs = { self, nixpkgs }:
     let
-      supportedSystems = [ "x86_64-linux" "aarch64-linux" ];
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
       forEachSystem = f: builtins.listToAttrs (map (system: {
         name = system;
         value = f system;
       }) supportedSystems);
     in {
       devShells = forEachSystem (system:
-        let pkgs = nixpkgs.legacyPackages.${system};
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+          # Tarball of pkgs.postgresql in the layout zonky's embedded-postgres expects
+          # (bin/, lib/, share/ at root). Used by the postgres scripted test on NixOS,
+          # where the bundled generic-Linux binaries can't run.
+          pgTarball = pkgs.runCommand "nomad-zonky-postgres.txz" {
+            nativeBuildInputs = [ pkgs.gnutar pkgs.xz ];
+          } ''
+            tar -cJf $out -C ${pkgs.postgresql} bin lib share
+          '';
         in {
           default = pkgs.mkShell {
             buildInputs = with pkgs; [ jdk21 sbt ];
             shellHook = ''
               export JAVA_HOME="${pkgs.jdk21}"
+              export NOMAD_PG_TARBALL="${pgTarball}"
             '';
           };
         }

sbt-plugin/src/sbt-test/nomad/clean-and-migrate-postgres/src/main/scala/Main.scala

@@ -3,7 +3,20 @@ import io.zonky.test.db.postgres.embedded.EmbeddedPostgres
 
 object Main {
   def main(args: Array[String]): Unit = {
-    val pg = EmbeddedPostgres.start()
+    // On NixOS zonky's bundled generic-Linux binaries fail to load. NOMAD_PG_TARBALL
+    // (set by the flake devshell) overrides the binary source with a tarball of the
+    // system postgres.
+    val pg = sys.env.get("NOMAD_PG_TARBALL").filter(_.nonEmpty) match {
+      case Some(path) =>
+        // nix-store postgres defaults unix_socket_directories to /run/postgresql,
+        // which doesn't exist in this sandbox — point it at the JVM temp dir instead.
+        EmbeddedPostgres
+          .builder()
+          .setPgBinaryResolver((_, _) => new java.io.FileInputStream(path))
+          .setServerConfig("unix_socket_directories", System.getProperty("java.io.tmpdir"))
+          .start()
+      case None => EmbeddedPostgres.start()
+    }
 
     try {
       val ds = pg.getPostgresDatabase()