chore: configure code quality tools

Add EditorConfig for consistent editor settings, Prettier for formatting (4-space indentation), ESLint with TypeScript rules, and linting documentation

LINTING.md

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+# Linting & Code Style Guide
+
+## Overview
+
+This project uses **ESLint** + **Prettier** with strict TypeScript rules and **4-space indentation**.
+
+## Quick Start
+
+```bash
+# Format all code
+pnpm format
+
+# Lint and auto-fix
+pnpm lint
+
+# Check only (no fix)
+pnpm eslint "packages/**/*.ts"
+```
+
+## Configuration Files
+
+- **`.prettierrc`** - Prettier code formatting (4 spaces, single quotes, etc.)
+- **`eslint.config.mjs`** - ESLint rules (TypeScript strict mode + best practices)
+- **`.editorconfig`** - Editor settings (consistent across IDEs)
+
+## Key Rules
+
+### TypeScript Strictness
+
+| Rule | Level | Description |
+|------|-------|-------------|
+| `@typescript-eslint/no-explicit-any` | warn | Avoid `any` type - use proper typing |
+| `@typescript-eslint/explicit-function-return-type` | warn | Functions must declare return types |
+| `@typescript-eslint/no-floating-promises` | error | Always handle promises |
+| `@typescript-eslint/no-unsafe-*` | warn | Strict type safety checks |
+
+### Code Quality
+
+| Rule | Level | Description |
+|------|-------|-------------|
+| `no-console` | warn | Use logger instead (except `console.warn/error`) |
+| `prefer-const` | error | Use `const` for non-reassigned variables |
+| `eqeqeq` | error | Always use `===` instead of `==` |
+| `curly` | error | Always use curly braces for if/for/while |
+| `complexity` | warn | Max cyclomatic complexity: 15 |
+| `max-params` | warn | Max function parameters: 5 |
+| `max-lines-per-function` | warn | Max 100 lines per function |
+
+### Code Style
+
+- **Indentation:** 4 spaces (not tabs)
+- **Line Length:** 100 characters max
+- **Quotes:** Single quotes `'string'`
+- **Semicolons:** Always required `;`
+- **Trailing Commas:** Always in multiline
+- **Arrow Functions:** Always use parentheses `(x) => x`
+
+## Handling Warnings
+
+### `any` Type Usage
+
+**Before:**
+```typescript
+function process(data: any) {
+    return data.value;
+}
+```
+
+**After:**
+```typescript
+interface ProcessData {
+    value: string;
+}
+
+function process(data: ProcessData): string {
+    return data.value;
+}
+```
+
+### Missing Return Types
+
+**Before:**
+```typescript
+async function fetchData() {
+    return await api.get('/data');
+}
+```
+
+**After:**
+```typescript
+async function fetchData(): Promise<DataResponse> {
+    return await api.get('/data');
+}
+```
+
+### Floating Promises
+
+**Before:**
+```typescript
+async function init() {
+    startServer(); // ❌ Promise not handled
+}
+```
+
+**After:**
+```typescript
+async function init(): Promise<void> {
+    await startServer(); // ✅ Promise awaited
+}
+```
+
+## Ignoring Rules (Use Sparingly)
+
+```typescript
+// Disable specific rule for next line
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+const legacyData: any = getLegacyApi();
+
+// Disable for entire file (add at top)
+/* eslint-disable @typescript-eslint/no-explicit-any */
+```
+
+**Note:** Only use `eslint-disable` for legacy code or when type safety is genuinely impossible.
+
+## Pre-commit Hooks (Optional)
+
+To automatically format on commit, install Husky:
+
+```bash
+pnpm add -D husky lint-staged
+npx husky init
+
+# Add to .husky/pre-commit:
+pnpm lint-staged
+```
+
+Create `lint-staged` config in `package.json`:
+
+```json
+{
+  "lint-staged": {
+    "*.{ts,tsx}": [
+      "prettier --write",
+      "eslint --fix"
+    ]
+  }
+}
+```
+
+## IDE Setup
+
+### VS Code
+
+Install extensions:
+- **ESLint** (`dbaeumer.vscode-eslint`)
+- **Prettier** (`esbenp.prettier-vscode`)
+
+Add to `.vscode/settings.json`:
+
+```json
+{
+  "editor.formatOnSave": true,
+  "editor.defaultFormatter": "esbenp.prettier-vscode",
+  "editor.codeActionsOnSave": {
+    "source.fixAll.eslint": true
+  },
+  "eslint.validate": ["typescript", "javascript"]
+}
+```
+
+### WebStorm / IntelliJ IDEA
+
+1. **Settings → Languages & Frameworks → JavaScript → Prettier**
+   - Enable "On save"
+   - Set "Run for files": `{**/*,*}.{ts,js,tsx,jsx}`
+
+2. **Settings → Languages & Frameworks → JavaScript → Code Quality Tools → ESLint**
+   - Enable "Automatic ESLint configuration"
+   - Enable "Run eslint --fix on save"
+
+## Common Issues
+
+### "Parsing error: ESLint was configured to run..."
+
+**Solution:** Make sure `tsconfig.json` exists in the project root.
+
+### "Rule 'indent' conflicts with Prettier"
+
+**Solution:** Already handled - ESLint `indent` rule is disabled in favor of Prettier.
+
+### Too many warnings after upgrade
+
+**Solution:** Run auto-fix first, then address remaining issues:
+
+```bash
+pnpm lint
+# Review warnings, fix manually
+```
+
+## Best Practices
+
+1. **Run `pnpm format` before committing** - ensures consistent formatting
+2. **Fix warnings incrementally** - don't disable rules without reason
+3. **Add types instead of `any`** - improves code quality and IDE support
+4. **Use ESLint ignore comments sparingly** - only for edge cases
+
+## Resources
+
+- [TypeScript ESLint Rules](https://typescript-eslint.io/rules/)
+- [Prettier Options](https://prettier.io/docs/en/options.html)
+- [ESLint Rules](https://eslint.org/docs/latest/rules/)
+- [Airbnb JavaScript Style Guide](https://github.com/airbnb/javascript)
+
+---
+
+**Last Updated:** 2025-01-19