Add Copilot instructions for repository

Co-authored-by: hckhanh <[email protected]>

Author: Copilot

.github/copilot-instructions.md

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+# Copilot Instructions for afetch
+
+## Project Overview
+
+This is **afetch**, a type-safe API client library with schema validation using Standard Schema. The project provides end-to-end type inference for API requests and responses, integrating with popular schema validation libraries like Zod, Valibot, and ArkType.
+
+## Installation & Setup
+
+```bash
+# Install dependencies
+npm install
+
+# Build the project
+npm run build
+
+# Run tests
+npm test
+
+# Format code
+npm run format
+```
+
+## Code Style & Standards
+
+### Formatter & Linter
+- **Biome** is used for both formatting and linting
+- Single quotes for strings
+- Semicolons only when needed (ASI style)
+- Arrow function parentheses always included
+- 2-space indentation
+- Run `npm run format` to auto-fix style issues
+
+### TypeScript Guidelines
+- Use strict TypeScript with full type safety
+- Leverage type inference wherever possible
+- Use `type` for type aliases (not `interface`) for consistency
+- Include JSDoc comments for public APIs
+- Use `.ts` extension in imports (e.g., `from './types.ts'`)
+- Prefer `const` over `let`
+- Use `unknown` instead of `any` where possible
+
+### Testing
+- Tests are located in the `test/` directory
+- Use Vitest as the test framework
+- Mock external dependencies (e.g., `global.fetch`)
+- Test file naming: `*.test.ts`
+- Include tests for:
+  - Happy paths
+  - Error cases
+  - Edge cases
+  - Type safety validations
+
+## Project Structure
+
+```
+src/
+  ├── index.ts       # Main export and createFetch function
+  ├── types.ts       # Type definitions
+  └── utils.ts       # Utility functions (e.g., validation)
+test/
+  └── createFetch.test.ts  # Test suite
+```
+
+## Key Dependencies
+
+- **@standard-schema/spec**: Standard Schema specification for validation
+- **fast-url**: URL building and parameter substitution
+- **vitest**: Testing framework
+- **biome**: Linting and formatting
+- **tsdown**: TypeScript bundler powered by rolldown
+
+## Development Workflow
+
+1. Make code changes in `src/`
+2. Add tests in `test/`
+3. Run `npm run format` to auto-format
+4. Run `npm test` to verify tests pass
+5. Run `npm run build` to ensure the build succeeds
+
+## API Design Principles
+
+- **Type Safety First**: All APIs must provide full type inference
+- **Standard Schema Compatible**: Work with any schema library implementing Standard Schema
+- **Minimal Surface Area**: Keep the API simple and focused
+- **Zero Breaking Changes**: Maintain backward compatibility
+- **Bundle Size**: Keep dependencies minimal and bundle size small
+
+## Common Tasks
+
+### Adding a New Feature
+1. Define types in `types.ts` if needed
+2. Implement functionality in `index.ts` or `utils.ts`
+3. Add JSDoc comments with examples
+4. Write comprehensive tests
+5. Update README.md if adding public APIs
+
+### Fixing a Bug
+1. Write a failing test that reproduces the bug
+2. Fix the bug in the source code
+3. Verify the test passes
+4. Add regression tests if needed
+
+### Refactoring
+1. Ensure all tests pass before starting
+2. Make incremental changes
+3. Run tests frequently
+4. Do not change public API signatures without careful consideration
+
+## Important Notes
+
+- This project uses **Bun** in CI but supports all package managers
+- The package is published to both npm and JSR (Deno)
+- The library must remain framework-agnostic
+- Response validation happens automatically when schemas are provided
+- Default method is GET; POST is used when body is present
+- Custom headers merge with default headers (Content-Type: application/json)