fix(sea-builder): add postject as direct devDependency

Add postject to node-sea-builder package.json to ensure it's available
via pnpm exec on all platforms, particularly Windows where PATH resolution
can differ from Linux/macOS.

This fixes Windows SEA builds failing with:
"postject is required to inject the SEA blob into the Node.js binary"

The dependency was already in the root package.json but wasn't being
resolved properly by pnpm exec postject on Windows runners.

Author: jdalton

packages/node-sea-builder/package.json

@@ -4,6 +4,7 @@
   },
   "description": "Native Node.js SEA binary builder (fallback)",
   "devDependencies": {
+    "postject": "1.0.0-alpha.6",
     "vitest": "3.0.0"
   },
   "license": "MIT",