Rename .js to .mjs, extract helpers module

Move resolvePath and exec out of index.mjs into server/helpers.mjs so
actions can import them directly rather than receiving them as arguments.

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

package.json

@@ -4,8 +4,8 @@
   "description": "A supervised action bridge between Claude Code and the host system",
   "type": "module",
   "scripts": {
-    "server": "node server/index.js",
-    "client": "node client/index.js"
+    "server": "node server/index.mjs",
+    "client": "node client/index.mjs"
   },
   "dependencies": {
     "express": "^5.2.1"