Add Copilot coding agent instructions

.github/copilot-instructions.md

@@ -0,0 +1,56 @@
+# Copilot Instructions for oak
+
+## Overview
+
+oak is a middleware framework for Deno's native HTTP server,
+[Deno Deploy](https://deno.com/deploy), Node.js, [Bun](https://bun.sh/), and
+[Cloudflare Workers](https://workers.cloudflare.com/). It is inspired by
+[Koa](https://github.com/koajs/koa/) and includes a middleware router inspired
+by [@koa/router](https://github.com/koajs/router/). The package is distributed
+via [JSR](https://jsr.io/@oak/oak).
+
+## Repository Structure
+
+- **Root-level `.ts` files** — Core framework modules (e.g., `application.ts`,
+  `router.ts`, `context.ts`, `request.ts`, `response.ts`, `body.ts`, `send.ts`).
+- **`middleware/`** — Built-in middleware (etag, proxy, serve).
+- **`utils/`** — Internal utility functions and type guards.
+- **`*.test.ts`** — Test files co-located with source files.
+- **`deps.ts`** — Centralized runtime dependencies (always import shared deps from here).
+- **`deps_test.ts`** — Centralized test-only dependencies.
+- **`mod.ts`** — Public entry point that re-exports the full public API.
+- **`types.ts`** — Shared TypeScript types and interfaces.
+- **`examples/`** — Example usage scripts (not published).
+- **`fixtures/`** — Test fixtures (not published).
+- **`docs/`** — Documentation site source (not published).
+
+## Development Commands
+
+All commands use the [Deno](https://deno.com/) toolchain (no `npm` or build step needed).
+
+| Task | Command |
+|---|---|
+| Run tests | `deno task test` |
+| Run tests with coverage | `deno task test:coverage` |
+| Generate lcov report | `deno task coverage` |
+| Format code | `deno fmt` |
+| Check formatting | `deno fmt --check` |
+| Lint code | `deno lint` |
+
+## Coding Conventions
+
+- **Language**: TypeScript, targeting Deno-first with cross-runtime support (Node.js, Bun).
+- **Copyright header**: Every source file begins with:
+  ```ts
+  // Copyright 2018-2025 the oak authors. All rights reserved. MIT license.
+  ```
+- **Imports**: Use relative imports with `.ts` extensions (e.g., `import { foo } from "./bar.ts"`).
+  Shared dependencies are imported via `deps.ts`; test-only dependencies via `deps_test.ts`.
+- **JSDoc**: Public APIs are documented with JSDoc. Module-level doc comments end with a `@module` tag.
+- **Formatting**: Enforced by `deno fmt`. Do not manually adjust whitespace or indentation.
+- **Linting**: Enforced by `deno lint`. The `no-import-prefix` rule is disabled (see `deno.json`).
+- **Tests**: Co-located `*.test.ts` files. Use `assertEquals`, `assertStrictEquals`, and
+  `assertRejects` from `deps_test.ts`. Mock helpers are defined locally within test files.
+- **Error handling**: Use the `errors` object from `deps.ts` for HTTP error classes.
+- **Exports**: All public symbols are re-exported from `mod.ts`. The `deno.json`
+  `exports` field also defines the package entry points for JSR publishing.

deno.json

@@ -38,7 +38,7 @@
     "test:coverage": "deno test --coverage=./cov --junit-path=junit.xml --allow-read --allow-write --allow-net --cert ./examples/tls/RootCA.crt --parallel --ignore=npm"
   },
   "fmt": {
-    "exclude": ["README.md"]
+    "exclude": ["README.md", ".github/copilot-instructions.md"]
   },
   "lint": {
     "rules": {