Switch build output to CommonJS (.cjs) format

Updated build script to output .cjs files instead of .js, and updated action.yml to reference the new .cjs files. Also clarified proxy environment variable handling in bootstrap.js for consistency.

package.json

@@ -8,7 +8,7 @@
     "node": ">=24.4.0"
   },
   "scripts": {
-  "build": "esbuild bootstrap.js main.js post.js --bundle --outdir=dist --format=esm --platform=node --packages=bundle",
+  "build": "esbuild bootstrap.js main.js post.js --bundle --outdir=dist --out-extension:.js=.cjs --platform=node --packages=bundle",
     "test": "c8 --100 ava tests/index.js",
     "coverage": "c8 report --reporter html",
     "postcoverage": "open-cli coverage/index.html"