Add GitHub Copilot instructions

As per discussion in https://github.com/orgs/PrivateBin/discussions/1696 here are some instructions generated by Copilot itself.

I used this prompt (in chat not agent mode, but well... should be enough) as suggested by GitHub: https://docs.github.com/copilot/how-tos/configure-custom-instructions/add-repository-instructions#anweisen-von-copilot-programmier-agent-zum-generieren-einer-copilot-instructionsmd-datei

Also added the feedback from the discussion.

.github/copilot-instructions.md

@@ -0,0 +1,118 @@
+# Copilot Coding Agent Onboarding for PrivateBin
+
+## Repository Overview
+
+**PrivateBin** is a minimalist, open-source online pastebin where the server has zero knowledge of the pasted data. All data is encrypted/decrypted in the browser via 256-bit AES (Galois Counter Mode). The project is a refactored fork of ZeroBin focused on extensibility and additional features.
+
+- **Main Use Case:** Secure, ephemeral sharing of text/code, with encryption happening exclusively client-side.
+- **Languages:** PHP (~48%), JavaScript (~35%), CSS (~17%), with some legacy browser support (see `legacy.js`).
+- **Type:** Web application (pastebin), with both server (PHP) and client (JavaScript) components.
+
+## Build & Validation Instructions
+
+### Prerequisites
+
+- **PHP:** 7.0+ (recommended: latest stable 7.x or 8.x)
+- **Composer:** For dependency management (`composer install`)
+- **Node.js & npm:** Only required for running JavaScript unit tests. **Main JS logic must remain browser-compatible!**
+- **Recommended Environment:** Unix-like OS (Linux, macOS) with Apache or Nginx for full demo.
+
+### Bootstrap & Install
+
+1. **Clone the repository** and enter its root directory.
+2. **Install PHP dependencies:**
+   ```sh
+   composer install
+   ```
+   - Always run this before building or testing PHP code.
+   - If you receive permission errors, verify `vendor/` is writable.
+
+3. **Install JS dependencies (for test only):**
+   ```sh
+   npm install
+   ```
+   - Only required for running JS tests. Not needed for building or running the app.
+
+### Build
+
+- **No explicit build step** for PHP. The web app is served directly from source.
+- **For JavaScript:** There is no webpack/bundler step for release; browser JS is written in compatible ES5+ syntax except in `legacy.js` (ES3).
+
+### Run
+
+- **PHP Server Mode:** Use Apache/Nginx with PHP, pointing the web root to the repo root or the `public/` directory (if present).
+- **Demo:** Open `index.php` in a browser (via your web server).
+
+### Test
+
+- **PHP Unit Tests:**
+  ```sh
+  vendor/bin/phpunit
+  ```
+  - Always run after code changes to backend logic.
+  - If `vendor/bin/phpunit` does not exist, ensure `composer install` completed without errors.
+
+- **JavaScript Unit Tests:**
+  ```sh
+  npm test
+  ```
+  - Runs Jasmine-based tests in Node.js context.
+  - Note: **Production JS must not use Node-only APIs.** Test code may use Node.js features, but main JS logic must remain browser-compatible.
+  - If you encounter `ReferenceError` for browser features, ensure only test code uses Node.js APIs.
+
+### Lint
+
+- **PHP:** Run (if `phpcs.xml` or similar config exists):
+  ```sh
+  vendor/bin/phpcs
+  ```
+- **JavaScript:** If `eslint` is present:
+  ```sh
+  npm run lint
+  ```
+  - Check for configuration in `.eslintrc.*` files.
+
+### Validation / CI
+
+- **GitHub Actions:** CI runs `composer install`, `phpunit`, and JS tests on PRs and pushes.
+- **Pre-commit:** Always run both PHP and JS tests before submitting PRs. Fix any warnings or errors.
+
+## Project Layout & Structure
+
+- **Root files:**
+  - `index.php`: Main server entry point.
+  - `README.md`: Project overview ([view full](https://github.com/PrivateBin/PrivateBin/blob/9d7508f44fac95dfadf4aad4fb3d3be128633336/README.md)).
+  - `composer.json`, `composer.lock`: PHP dependencies.
+  - `package.json`: JS test/lint dependencies (not for production JS).
+  - `.github/workflows/`: CI configuration.
+  - `cfg/`: Default configuration files.
+  - `js/`: Main client logic (browser JS), including:
+    - `legacy.js`: Must remain compatible with legacy browsers (ES3). **Do not use modern JS here.**
+    - `main.js`: Core encryption and paste interface logic.
+  - `tpl/`: HTML templates.
+  - `css/`: Stylesheets.
+
+- **Testing & Validation:**
+  - `test/`: Contains PHP and JS unit tests.
+  - `phpunit.xml`: PHPUnit config.
+  - JS test files may use Node.js features; browser JS must not.
+
+- **Key architectural notes:**
+  - **Encryption:** Only client-side in JS.
+  - **Backend:** Serves encrypted blobs, never sees plaintext.
+  - **Legacy Support:** `js/legacy.js` must remain ES3 for feature detection in old browsers.
+  - **Configuration:** See `cfg/conf.php` and wiki for available options.
+
+## Automated Checks
+
+- **GitHub CI:** On PRs, runs `composer install`, `phpunit`, and JS tests.
+- **Validation Steps:** PRs failing tests will be blocked. Always ensure a clean test run before submitting.
+
+## Guidance for Copilot Agent
+
+- **Trust these instructions.** Only perform a search if information is missing or appears incorrect.
+- **Do NOT use Node.js APIs in production JS code.** Only test code may do so.
+- **Never modernize `legacy.js`.** It must work in very old browsers.
+- **Always run `composer install` before PHP tests, and `npm install` before JS tests.**
+- **Validate all changes by running both PHP and JS tests.**
+- **Review `.github/workflows/` for the latest validation pipeline steps.**