Add workspace instructions for Copilot

.github/copilot-instructions.md

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+# Project Guidelines
+
+## Overview
+
+React expense tracker starter app (Vite + React 19). Intentionally contains bugs, poor UI, and messy code — designed as a teaching project for incremental refactoring.
+
+## Build and Test
+
+```bash
+npm install           # Install dependencies
+npm run dev           # Dev server → http://localhost:5173
+npm run build         # Production build → dist/
+npm run lint          # ESLint check
+```
+
+> **Windows note:** Use `npm.cmd` instead of `npm` if PowerShell execution policy blocks scripts.
+
+## Architecture
+
+- **Single-file app** — all logic lives in `src/App.jsx` (~180 lines)
+- **No component extraction** — summary cards, form, filters, and transaction table are all inline
+- **State:** multiple `useState` calls (no context, no external state library)
+- **Styling:** plain CSS files (`App.css`, `index.css`), kebab-case class names, no CSS modules
+
+## Known Issues (Intentional)
+
+These are learning targets, not legacy debt:
+
+1. **Amounts stored as strings** — causes string concatenation instead of arithmetic in totals
+2. **Missing table column** — `Type` header exists but no matching `<td>` in rows
+3. **Data error** — "Freelance Work" marked as expense, should be income
+4. **No delete feature** — `.delete-btn` CSS exists but no button in DOM
+5. **Monolithic component** — everything in one file, no extraction
+
+## Conventions
+
+- **Components:** PascalCase filenames, `.jsx` extension
+- **Variables:** camelCase
+- **CSS classes:** kebab-case
+- **Imports:** relative paths, CSS imported at component level
+- **State:** React `useState` hooks (keep consistent with existing pattern unless refactoring)
+
+## Refactoring Targets
+
+When asked to improve the code, consider these extraction candidates:
+- `<TransactionTable>`, `<AddTransactionForm>`, `<SummaryCard>` components
+- `useTransactions` custom hook for transaction logic
+- Constants file for hardcoded categories