Turbo Module Implementation

1. Generates turbo.json from stackpanel.tasks
2. Compiles task scripts with `exec` to Nix derivations
3. Creates symlinks in .tasks/bin/ for Turborepo to invoke
4. Generates package.json script entries
5. Handles per-app task overrides via stackpanel.apps.*.tasks

- Tasks with `exec` become writeShellApplication derivations
- Derivations are symlinked to .tasks/bin/<task>
- package.json scripts call ./.tasks/bin/<task>
- turbo.json references task names with deps/outputs/caching

stackpanel.tasks = {
  build = {
    exec = "npm run compile";
    after = [ "deps" "^build" ];
    outputs = [ "dist/**" ];
    runtimeInputs = [ pkgs.nodejs ];
  };
  dev = {
    persistent = true;
    cache = false;
  };
};