[Repo Assist] docs: add CONTRIBUTING.md guide for new contributors

CONTRIBUTING.md

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+# Contributing to vscode-phpcbf
+
+Thank you for considering contributing! This is a VS Code extension that runs [PHP Code Beautifier and Fixer (PHPCBF)](https://github.com/PHPCSStandards/PHP_CodeSniffer) on PHP files.
+
+## Getting started
+
+1. **Fork** the repository and clone your fork.
+2. Install dependencies:
+   ```bash
+   npm install
+   ```
+3. Open the project in VS Code. Press **F5** to launch a new Extension Development Host window with the extension loaded.
+
+## Project layout
+
+```
+extension.js      Main extension entry point — PHPCBF class and activation logic
+lib/utils.js      Pure utility functions (findFiles, etc.)
+test/
+  unit.test.js    Unit tests (no VS Code required) — run with: npm run test:unit
+  extension.test.js  VS Code integration tests — run via F5 + test runner
+package.json      Extension manifest (contributes, activationEvents, settings)
+CHANGELOG.md      Release notes
+```
+
+## Running tests
+
+### Unit tests (fast, no VS Code needed)
+
+```bash
+npm run test:unit
+```
+
+Requires Node.js 18+.
+
+### VS Code integration tests
+
+Press **F5** in VS Code to open the Extension Development Host, then run the test suite from there. These tests require a display environment and cannot run headlessly in most CI setups.
+
+## Code style
+
+- Plain JavaScript (ES6, CommonJS modules) — no TypeScript compilation step.
+- 4-space indentation; match the style of the surrounding code.
+- Run ESLint before submitting: `./node_modules/.bin/eslint extension.js lib/`
+- Keep changes focused — one logical change per PR.
+
+## Submitting a pull request
+
+1. Create a feature branch off `master`: `git checkout -b fix/description`.
+2. Make your changes and add or update tests where possible.
+3. Run `npm run test:unit` to confirm tests pass.
+4. Open a pull request against `master` with a clear title and description.
+5. Reference any related issues using `Closes #N` or `Related to #N`.
+
+## Reporting bugs
+
+Please include:
+- VS Code version and OS
+- The `phpcbf.executablePath` you are using (output of `which phpcbf` or equivalent)
+- Whether `phpcbf.debug: true` output shows anything useful (View → Output → phpcbf)
+- Steps to reproduce
+
+## Common configuration tips
+
+| Problem | Setting to try |
+|---------|---------------|
+| Extension can't find `phpcbf` | Set `phpcbf.executablePath` to the absolute path |
+| phpcs.xml not picked up | Enable `phpcbf.configSearch: true` |
+| Relative path in `phpcbf.standard` fails | Use `"${workspaceFolder}/phpcs.xml"` |
+| Formatting doesn't run on save | Add `"[php]": { "editor.defaultFormatter": "soderlind.phpcbf", "editor.formatOnSave": true }` |
+
+## Notes for maintainers
+
+- `package.json` changes (version bumps, new settings) must be made manually as they are in a protected branch context for automated workflows.
+- The extension ID on the Marketplace is `persoderlind.phpcbf` (note the `per` prefix).