Add .github/copilot-instructions.md (#452)

* Initial plan

* Add .github/copilot-instructions.md with repository-specific guidance

Co-authored-by: mikeharder <9459391+mikeharder@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: mikeharder <9459391+mikeharder@users.noreply.github.com>
Co-authored-by: Mike Harder <mharder@microsoft.com>

Author: Copilot

.github/copilot-instructions.md

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+# Copilot Instructions
+
+## Project Overview
+
+This repository contains the source code for `openapi-diff` (aka `oad`, aka `@azure/oad`), a breaking-change
+detector tool for OpenAPI specifications. It is published as an npm package and is used internally by the
+[azure-rest-api-specs](https://github.com/Azure/azure-rest-api-specs) and
+[azure-rest-api-specs-pr](https://github.com/Azure/azure-rest-api-specs-pr) repos to validate PRs.
+
+The core diff logic is written in C#/.NET and the CLI/validation layer is written in TypeScript.
+
+## Tech Stack
+
+- **TypeScript** (ES2017 target, CommonJS modules) — CLI, validation, and test code in `src/`
+- **C#/.NET 6** — core diff engine in `openapi-diff/`
+- **Node.js 20+**
+- **Jest** with `ts-jest` for TypeScript tests
+- **ESLint** (`typescript-eslint`) for linting
+- **Prettier** for formatting
+- **markdownlint** for Markdown style
+
+## Setup
+
+```sh
+npm ci                # Install Node.js dependencies
+npm run dn.restore    # Restore .NET dependencies (if working on C# code)
+```
+
+## Build
+
+```sh
+npm run tsc           # Build TypeScript code (outputs to dist/)
+npm run dn.build      # Build C# code
+```
+
+## Lint and Format
+
+```sh
+npm run lint          # Run ESLint (zero warnings allowed)
+npm run lint:fix      # Auto-fix ESLint issues
+npm run prettier      # Check formatting with Prettier
+npm run prettier:write # Auto-fix formatting
+```
+
+## Test
+
+```sh
+npm run ts.test       # Run TypeScript tests (Jest)
+npm run dn.test       # Run C# tests (dotnet test)
+npm test              # Run both C# and TypeScript tests
+```
+
+TypeScript tests live in `src/test/` and follow the naming pattern `*[tT]est.ts`.
+
+## Code Conventions
+
+- **No semicolons** — Prettier is configured with `"semi": false`
+- **Print width** is 140 characters
+- **Trailing commas** are not used (`"trailingComma": "none"`)
+- **Arrow function parentheses** are avoided when possible (`"arrowParens": "avoid"`)
+- **Strict TypeScript** — `tsconfig.json` has `"strict": true`
+- **ESLint config** is in `eslint.config.js` (CommonJS format, not ESM)
+- **Markdown line length** limit is 120 characters (tables and headings are exempt)
+- Prefer `const` over `let` where possible
+
+## Repository Structure
+
+```
+src/
+  cli.ts              # CLI entry point
+  index.ts            # Package entry point
+  lib/
+    commands/          # Command implementations
+    util/              # Utility modules
+    validate.ts        # Validation logic
+    validators/        # Validator implementations
+  test/                # TypeScript tests (Jest)
+    specs/             # Test fixture OpenAPI spec files
+openapi-diff/          # C#/.NET solution (core diff engine)
+docs/                  # Documentation
+```
+
+## CI
+
+CI runs on both Ubuntu and Windows via GitHub Actions (`.github/workflows/ci.yml`). It runs:
+`npm run lint`, `npm run prettier`, and `npm test` (which includes both C# and TypeScript tests).
+
+## Guidelines
+
+- Run `npm run lint` and `npm run prettier` before submitting changes.
+- Add or update tests in `src/test/` for any TypeScript code changes.
+- Do not modify CI/CD pipeline files (`.github/workflows/`, `azure-pipelines.yml`) unless specifically required.
+- When working only on TypeScript, use `npm run ts.test` to run just the TypeScript tests.