Fix action runtime by bundling dependencies with ncc

The action was failing with "Cannot find module '@actions/core'" because
node_modules weren't available at runtime. This commit fixes the issue by:

- Adding @vercel/ncc to bundle all dependencies into a single dist/index.js file
- Updating action.yml to point to the bundled dist/index.js
- Adding a build script to compile the action
- Including the dist/ directory in the repository (standard for GitHub Actions)

This is the recommended approach for JavaScript-based GitHub Actions to ensure
all dependencies are available at runtime without requiring npm install.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

package.json

@@ -4,6 +4,7 @@
   "description": "A minimal GitHub Action implemented in Node.js",
   "main": "index.js",
   "scripts": {
+    "build": "ncc build index.js -o dist",
     "test": "jest",
     "test:watch": "jest --watch",
     "test:coverage": "jest --coverage"
@@ -20,7 +21,8 @@
     "@actions/exec": "^1.1.1"
   },
   "devDependencies": {
-    "jest": "^29.7.0"
+    "jest": "^29.7.0",
+    "@vercel/ncc": "^0.38.1"
   },
   "jest": {
     "testEnvironment": "node",