fix: switch to @vercel/ncc for proper dependency bundling (#8)

## Summary
- Replace tsdown with @vercel/ncc to properly bundle all dependencies
- Fixes "Cannot find module '@actions/core'" error when action runs in
GitHub workflows
- Updates lint-staged configuration to work with ncc build process

## Problem
The previous bundler (tsdown) wasn't including node_modules dependencies
in the output, causing the action to fail with module resolution errors
when executed in GitHub Actions environments.

## Solution
Switched to @vercel/ncc which is the standard and recommended bundler
for GitHub Actions. It creates a self-contained JavaScript file with all
dependencies included.

## Test plan
- [x] All existing tests pass (`pnpm run validate`)
- [x] Bundle loads successfully when tested with Node.js
- [x] Bundle contains all required functions from @actions/core and
@actions/github
- [x] Lint-staged hooks work correctly with the new build configuration

Author: dcramer

dist/cli.d.ts

@@ -1,4 +1,4 @@
-require('./sourcemap-register.js');/******/ (() => { // webpackBootstrap
+/******/ (() => { // webpackBootstrap
 /******/ 	var __webpack_modules__ = ({
 
 /***/ 3191:
@@ -41123,5 +41123,4 @@ exports.VERSION = '5.9.2'; // x-release-please-version
 /******/ 	module.exports = __webpack_exports__;
 /******/ 	
 /******/ })()
-;
-//# sourceMappingURL=index.js.map
+;