C2(jobsync): impl — add /usr/bin/env symlink for npx-installed scripts

npx-downloaded prisma has `#!/usr/bin/env node` shebang. Nix containers
lack FHS paths; create the symlink in extraCommands.

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

containers/jobsync/default.nix

@@ -97,9 +97,10 @@ pkgs.dockerTools.buildLayeredImage {
     pkgs.coreutils
   ];
 
-  # Create writable directories that don't exist in nix containers
+  # Create writable directories and FHS symlinks for nix container
   extraCommands = ''
-    mkdir -p tmp data
+    mkdir -p tmp data usr/bin
+    ln -s ${pkgs.coreutils}/bin/env usr/bin/env
   '';
 
   config = {