docs: Create comprehensive GitHub Copilot instructions for System.IO.Abstractions development

[Copilot]

This PR creates a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed guidance on how to work effectively in the System.IO.Abstractions codebase.

Key Features

Essential Setup & Build Instructions:

• Exact .NET SDK 9.0.304 installation commands (required by global.json)
• Validated build commands with measured timing (~70s build, ~30s tests)
• Critical "NEVER CANCEL" warnings with appropriate timeout recommendations (120+ minutes for builds, 60+ minutes for tests)
• Code formatting requirements using dotnet format with validation steps

Project Structure Guidance:

• Clear explanation of the 4 main projects: abstractions, wrappers, testing helpers, and multiple test projects
• Multiple target framework support (net472, netstandard2.0/2.1, net6.0/8.0/9.0)
• Important directory structure and build artifact locations

Validation Scenarios:

• Complete manual validation checklist for verifying changes
• Working code example demonstrating both MockFileSystem and real FileSystem usage:

var mockFileSystem = new MockFileSystem();
mockFileSystem.File.WriteAllText(@"C:\test.txt", "Test content");
var content = mockFileSystem.File.ReadAllText(@"C:\test.txt");

var realFileSystem = new FileSystem();
// ... real filesystem operations

Pull Request Guidelines:

• Conventional commit prefixes for communicating intent to library consumers
• Standard prefixes: fix:, feat:, refactor:, docs:, chore:

Performance & Timing:

• All commands validated with actual execution times during development
• Explicit timeout recommendations based on measured performance
• Clear warnings about long-running operations to prevent premature cancellation

Troubleshooting Section:

• Common issues (GitVersion errors, shallow clone limitations, platform-specific test failures)
• Solutions for SDK installation problems
• Alternative approaches when NUKE build script fails

The instructions follow an imperative tone ("Run this command", "Do not do this") and prioritize practical, validated commands that developers can copy-paste and have work immediately. Every command and timing recommendation was thoroughly tested during the creation of these instructions.

Fixes #1327.

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[vbreuss]

@copilot add a section to use a correct pull request title:

Pull Request Title

To communicate intent to the consumers of your library, the title of the pull requests is prefixed with one of the following elements:

• fix:: patches a bug
• feat:: introduces a new feature
• refactor: improves internal structure without changing the observable behavior
• docs: updates documentation or XML comments
• chore: updates to dependencies, build pipelines, ...

[Copilot]

@copilot add a section to use a correct pull request title:

Pull Request Title

To communicate intent to the consumers of your library, the title of the pull requests is prefixed with one of the ...

Added the Pull Request Title guidelines section with conventional commit prefixes as requested. The section includes all the specified prefixes (fix:, feat:, refactor:, docs:, chore:) to help communicate intent to library consumers. Commit d300d7d.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Test Results

   22 files  ±0     22 suites  ±0   10s ⏱️ ±0s
1 139 tests ±0  1 138 ✅ ±0    1 💤 ±0  0 ❌ ±0 
7 610 runs  ±0  7 048 ✅ ±0  562 💤 ±0  0 ❌ ±0 

Results for commit d300d7d. ± Comparison against base commit 2fdecff.

[github-actions]

This is addressed in release v22.0.16.