Add GitHub Copilot instructions for SSH.NET

[Copilot]

Adds .github/copilot-instructions.md to give GitHub Copilot accurate, repository-specific context so AI-generated contributions align with existing architecture, style, and safety expectations.

Coverage

• Domain & entry points — SSH-2 library capabilities; SshClient, SftpClient, ScpClient, NetConfClient all extending BaseClient
• Core technologies — C# LangVersion=latest, multi-TFM (.NET 4.6.2 / netstandard2.0 / .NET 8+), BouncyCastle for crypto, MEL abstractions for logging, MSTest + Moq + Testcontainers for testing, StyleCop + Meziantou + Sonar analyzers
• Code organization — namespace/directory map, partial-class pattern for large files (e.g. PrivateKeyFile.PKCS1.cs, PrivateKeyFile.OpenSSH.cs)
• Naming & style — PascalCase members, _camelCase private fields, I-prefix interfaces; StyleCop rules: #nullable enable at file top, using directives outside namespace with System.* first, XML doc on all public/internal members
• Error handling — full SshException hierarchy; #if NETFRAMEWORK serialization constructor pattern; ErrorOccurred events for cross-thread errors; no silent swallowing
• API design — every blocking method needs an …Async(CancellationToken) counterpart; ReadOnlyMemory<byte> preferred over byte[] in protocol code; no exposed mutable collections
• Async & concurrency — ConfigureAwait(false) throughout; TaskCompletionSource<T> for event bridging; no .Result/.Wait() inside async code
• Security-sensitive areas — key exchange (Security/), private key parsing (PrivateKeyFile*.cs), host key verification; guidance to clear key material and never log secrets; WiresharkKeyLogFilePath must not be enabled in production
• Testing — MSTest Arrange-Act-Assert pattern, file-per-scenario partial class layout, test commands including coverage collection
• Performance — ArrayPool/MemoryPool over ad-hoc byte[], lock-contention avoidance, BenchmarkDotNet for significant changes
• Safe-change checklist — unit test baseline, multi-TFM build verification, breaking-change policy, Release-mode analyzer enforcement

Original prompt

Create a pull request in sshnet/SSH.NET targeting the develop branch that adds repository-specific GitHub Copilot instructions based on analysis of the existing codebase.

Goal

Add a Copilot instruction document tailored to this repository so AI-generated code aligns with project architecture, coding style, testing approach, and contribution expectations.

Required work

1. Analyze the repository structure and code patterns across the project.
2. Infer and document:
   • What the repository does (domain and major capabilities).
   • Core technologies used (language/runtime/tooling/frameworks).
   • Architectural and code organization patterns.
   • Naming/style conventions used in the code.
   • Error handling and API design patterns.
   • Testing strategy and expectations.
   • Performance/concurrency considerations relevant to this repo.
   • Security-sensitive areas and guidance for safe changes.
   • Documentation/change hygiene expectations for contributors.
3. Create a Copilot instruction file suitable for GitHub Copilot in this repository (use the currently recommended location/format, e.g. .github/copilot-instructions.md).
4. Ensure instructions are practical and actionable, with concise “do/don’t” guidance and examples where helpful.
5. Keep guidance repository-specific; avoid generic boilerplate unless necessary.
6. If appropriate, include a short section on how to propose changes safely in this repo (tests to run, compatibility considerations, and scope discipline).

Quality bar

• Instructions should clearly reflect real patterns found in this repository.
• The document should be easy for contributors and Copilot to follow.
• Keep tone professional and concise.
• Ensure markdown formatting is clean.

Deliverables

• New Copilot instruction file committed in the PR.
• PR description summarizing the repository analysis and what guidance was added.

This pull request was created from Copilot chat.

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