ENG-219: Use .js extension for tsdown ESM output

Set fixedExtension: false so tsdown outputs .js instead of .mjs,
since the project already has "type": "module" in package.json.
This keeps the bin path at ./dist/cli.js matching what tsup produced.

Author: d4mation

tsdown.config.ts

@@ -5,6 +5,7 @@ export default defineConfig({
   format: ['esm'],
   target: 'node24',
   outDir: 'dist',
+  fixedExtension: false,
   sourcemap: true,
   dts: false,
   outputOptions: {