Add project guidelines and best practices to Copilot instructions

Author: kunman93

.github/copilot-instructions.md

@@ -0,0 +1,25 @@
+# Project Guidelines
+
+## Technology Stack
+
+- Node.js with Express
+- SQLite via better-sqlite3
+- EJS templates for server-rendered HTML
+- xUnit-style tests with Jest
+
+## Code Style
+
+- Use async/await — never raw callbacks or `.then()` chains
+- Follow the existing error handling pattern in `src/errorHandler.js`
+- All new routes must be added to `src/routes.js` following the existing pattern
+- Use 2-space indentation
+
+## Testing
+
+- Every new route must have a corresponding test in `tests/`
+- Use supertest for HTTP integration tests
+
+## Security
+
+- Never log user-provided input directly
+- Validate all request parameters before database access