Convert the action to ESM to consume @actions/core 3 and @actions/github 9

[jeefy]

@actions/core 3.x and @actions/github 9.x are ESM-only: their exports maps expose only an import condition. Our build is CommonJS (tsconfig module: commonjs, ncc emits CJS), so webpack fails with

"." is not exported under the conditions ["require","node","production"] from package @actions/core

and skipLibCheck does not help — it is a bundling constraint, not a type-checking one. #123 therefore landed on the last CommonJS majors (@actions/github 8.0.1, @actions/core 2.0.3), which are current-generation in every other respect (octokit core 7, http-client 3, undici 6).

To go further the action itself needs to become ESM:

• package.json "type": "module"; tsconfig module/moduleResolution → nodenext
• Replace the 26 import type { Context } from '@actions/github/lib/context' (subpath not in the v9 exports map) with a local alias such as type Context = typeof github.context, and WebhookPayload with typeof github.context.payload
• __tests__/testUtils.ts MockContext extends Context needs the class via github.context.constructor
• __tests__/bundle/runBundle.ts uses __dirname → import.meta.dirname
• Confirm ncc emits an ESM bundle and action.yml (node24) runs dist/index.js as ESM given the root package.json type
• Re-run the bundle harness; check for webpackMissingModule

Dependabot will keep proposing the v3/v9 bumps; close them referencing this issue until it is done.