Add CLAUDE.md with project reference guide

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CLAUDE.md

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+# SG Cars Trends API - Reference Guide
+
+## Commands
+- Build: `pnpm build`
+- Lint: `pnpm lint` (uses Biome)
+- Test all: `pnpm test`
+- Test watch: `pnpm test:watch`
+- Test coverage: `pnpm test:coverage`
+- Run single test: `pnpm -F @sgcarstrends/api test -- src/utils/__tests__/slugify.test.ts`
+- Package-specific test: `pnpm -F @sgcarstrends/<package> test`
+
+## Code Style
+- TypeScript with strict type checking
+- Use double quotes for strings (Biome enforced)
+- Use spaces for indentation (Biome enforced)
+- Organize imports automatically (Biome enforced)
+- Function/variable naming: camelCase
+- Class naming: PascalCase
+- Error handling: Use try/catch for async operations
+- Use workspace imports for shared packages: `@sgcarstrends/utils`, etc.
+- Path aliases: Use `@api/` for imports in api app
+
+## Testing
+- Testing framework: Vitest
+- Tests should be in `__tests__` directories next to implementation
+- Test file suffix: `.test.ts`