Modernize toolchain and improve test coverage

.github/copilot-instructions.md

@@ -0,0 +1,53 @@
+# Copilot Instructions
+
+## Architecture
+
+This is a **GitHub Action** (Node.js, TypeScript) that detects CRLF line endings in pull request files and blocks merges. It runs on `pull_request_target` events.
+
+**Flow:** `src/index.ts` (entry point) uses `@actions/github` to fetch PR files via the GitHub API, passes them to `src/validation.ts` which downloads each file's base64 content and checks for `\r\n`. If CRLF is found, the action posts a PR comment with fix instructions, adds a `crlf detected` label, and calls `core.setFailed()`.
+
+**Key modules:**
+
+- `src/index.ts` — Action entry point, orchestrates GitHub API calls
+- `src/validation.ts` — Core logic: file content checking, exclusion matching (minimatch), PR comment generation
+- `src/types.ts` — TypeScript types derived from `@octokit/openapi-types`
+- `src/strings.ts` — User-facing string constants for PR comments
+
+The `dist/` directory contains the bundled action output (committed to the repo) produced by Rollup.
+
+## Build, Test, and Lint
+
+```bash
+# Build (TypeScript compilation)
+npm run build
+
+# Run all tests
+npm run test
+
+# Run a single test by name
+npx cross-env NODE_OPTIONS='--experimental-vm-modules' jest -t "File with CRLF is detected properly"
+
+# Lint
+npm run lint
+
+# Bundle for distribution (must be committed)
+npm run package
+
+# Full pipeline (format + build + test + package)
+npm run all
+```
+
+After making changes, run `npm run package` and commit the updated `dist/` directory.
+
+## Conventions
+
+- **ESM modules** — The project uses `"type": "module"` with `.js` extensions in import paths (e.g., `'./types.js'`).
+- **Copyright header** — Every `.ts` and `.js` file must start with:
+  ```
+  // Copyright (c) Microsoft Corporation.
+  // Licensed under the MIT license.
+  ```
+  This is enforced by ESLint (`@tony.ganchev/eslint-plugin-header`).
+- **Prettier** — Single quotes, auto line endings. Integrated via ESLint.
+- **Tests** — Jest with `ts-jest` in ESM mode. Tests live in `test/` and mock Octokit HTTP calls by injecting a `jest.fn()` via Octokit's `request: { fetch }` option with native `Response` objects. Test files match `**/*.test.ts`.
+- **Unused variables** — Prefix with `_` (enforced by `@typescript-eslint/no-unused-vars`).

.github/workflows/node.yml

@@ -19,6 +19,6 @@ jobs:
     - name: Verify npm all runs and build output matches
       uses: actions/setup-node@v6
       with:
-        node-version: '20'
+        node-version: '24'
     - run: npm ci
     - run: npm run all

.gitignore

@@ -104,4 +104,5 @@ typings/
 .tern-port
 
 # TSC output
-lib
+dist/*
+!dist/index.js

action.yml

@@ -8,5 +8,5 @@ inputs:
     description: 'Custom exclusion list. Semi-colon delimited list of glob patterns.'
     required: false
 runs:
-  using: 'node20'
+  using: 'node24'
   main: 'dist/index.js'