Add .github/copilot-instructions.md

[Copilot]

Configure repository instructions for GitHub Copilot coding agent per best practices.

Covers:

• Project context — dual TypeScript + C#/.NET codebase, @azure/oad npm package
• Build/test/lint commands — npm run tsc, npm run ts.test, npm run lint, npm run prettier, etc.
• Code conventions — no semicolons, 140 char print width, no trailing commas, strict TS, CommonJS ESLint config
• Repo structure — src/ (TS), openapi-diff/ (C#), src/test/ (Jest, *[tT]est.ts pattern)
• CI — GitHub Actions on Ubuntu + Windows: lint, prettier, full test suite
• Guidelines — pre-submit checks, test expectations, files to avoid modifying

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• gh.io
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node /home/REDACTED/work/_temp/ghcca-node/node/bin/node --enable-source-maps /home/REDACTED/work/_temp/copilot-developer-action-main/dist/index.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #451

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[Copilot]

Pull request overview

Adds repository-specific Copilot Coding Agent instructions to help contributors/agents work effectively in this mixed TypeScript + C#/.NET codebase and follow existing build/lint/test conventions.

Changes:

• Introduces .github/copilot-instructions.md documenting project context, repo structure, and code conventions.
• Documents key npm scripts for build/test/lint/format and notes CI expectations.
• Captures TypeScript/C# stack details and testing file naming conventions.

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💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.