Configure Copilot coding agent instructions for SharpCompress

[Copilot]

Adds project-specific instructions for the GitHub Copilot coding agent to guide contributions to SharpCompress, a pure C# compression library supporting multiple archive formats.

Changes

AGENTS.md - Enhanced with SharpCompress-specific context:

• Archive format support matrix (Zip, Tar, 7Zip, Rar, LZip, XZ, ZStandard)
• Stream disposal patterns (v0.21+ behavior: wrapped streams closed by default, use LeaveStreamOpen or NonDisposingStream to override)
• Async/await patterns with CancellationToken for I/O operations
• Archive API (seekable, random access) vs Reader/Writer API (forward-only streaming) usage guidance
• Format-specific constraints (Tar requires file size in header, Zip64 needs seekable streams, encrypted LZMA unsupported)
• Testing guidelines for multi-format compression scenarios
• CSharpier formatting configuration (local tool in .config/dotnet-tools.json)

.github/COPILOT_AGENT_README.md - Corrected references:

• Fixed filename reference (AGENTS.md not AGENTS.yml)
• Added .github/agents/copilot-agent.yml to file listing
• Corrected test project name (SharpCompress.Test not SharpCompress.Tests)

Example Usage

The instructions guide agent to properly handle stream disposal:

// Correct: Explicit control over stream lifecycle
using (var fileStream = File.OpenRead("archive.zip"))
using (var reader = ReaderFactory.Open(fileStream, new ReaderOptions { LeaveStreamOpen = true }))
{
    // Process entries
}
// fileStream still open for further use

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:

• docs.github.com
  • Triggering command: curl -L REDACTED (dns block)
• gh.io
  • Triggering command: curl -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 #982

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