Add Copilot instructions for repository

[Copilot]

Configures GitHub Copilot instructions per best practices documentation to provide context-aware code assistance.

Changes

• Added .github/copilot-instructions.md with:
  • Project architecture: single-file Go CLI that converts markdown to HTML and launches in browser
  • Build matrix: cross-compilation for Linux (amd64/arm64/i386), Windows, macOS, FreeBSD
  • Conventions: version injection via ldflags, check() error handling pattern, //go:embed for resources
  • Platform-specific handling: Snap sandboxing constraints, MDVIEW_DIR environment override
  • Package targets: deb, snap, tarballs (tar.gz/zip)
  • Common tasks: flag addition workflow, HTML/CSS modifications, platform support expansion

File provides Copilot with repository context including Go 1.21 idioms, Makefile build patterns, and constraints like no existing test suite.

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: /usr/bin/curl curl -s REDACTED (dns block)
  • Triggering command: /usr/bin/curl curl -L -s REDACTED (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 #50

---

💡 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

This PR adds GitHub Copilot instructions to provide context-aware code assistance for the mdview repository. The instructions document follows best practices to help Copilot understand the project's architecture, conventions, and constraints.

• Added comprehensive Copilot instructions covering project structure, build system, and coding conventions
• Documented Go 1.21 idioms, cross-platform build patterns, and platform-specific handling requirements
• Included common development tasks and workflows to guide future code modifications