Set up Copilot instructions (#270)

+semver:patch

---------

Co-authored-by: Alex Zaytsev <hazzik@gmail.com>

Author: Copilot

.github/copilot-instructions.md

@@ -0,0 +1,72 @@
+# DelegateDecompiler - GitHub Copilot Instructions
+
+## Project Overview
+
+DelegateDecompiler is a .NET library that decompiles delegates and method bodies to their lambda representation. It enables computed properties in LINQ queries by translating `[Computed]` attributes into executable expressions that can be processed by Entity Framework and other ORMs.
+
+## Core Components
+
+- **`ComputedAttribute`**: Marks properties/methods for decompilation
+- **`MethodBodyDecompiler`**: Core IL decompilation engine  
+- **`DecompileExtensions`**: Public API for decompiling delegates/queries
+- **`Processors/`**: Pluggable processors for different IL opcodes
+
+## Build & Test
+
+```bash
+# Restore packages
+dotnet restore
+
+# Build the solution
+dotnet build --no-restore -c Release -p:DisableGitVersionTask=true
+
+# Run tests
+dotnet test --no-build -c Release -f net8.0 src/DelegateDecompiler.Tests
+dotnet test --no-build -c Release -f net8.0 src/DelegateDecompiler.Tests.VB
+dotnet test --no-build -c Release -f net8.0 src/DelegateDecompiler.EntityFramework.Tests
+dotnet test --no-build -c Release -f net8.0 src/DelegateDecompiler.EntityFrameworkCore6.Tests
+dotnet test --no-build -c Release -f net8.0 src/DelegateDecompiler.EntityFrameworkCore8.Tests
+dotnet test --no-build -c Release -f net9.0 src/DelegateDecompiler.EntityFrameworkCore9.Tests
+```
+
+**Development Guidelines:**
+- Use test-first approach when adding new features or fixing bugs
+- Ensure core unit tests pass: `src/DelegateDecompiler.Tests` and `src/DelegateDecompiler.Tests.VB` should have no failures
+- Run unit tests frequently during development to catch regressions early
+
+## Project Structure
+
+```
+src/
+├── DelegateDecompiler/                          # Core library
+├── DelegateDecompiler.EntityFramework/          # EF 6.x support
+├── DelegateDecompiler.EntityFrameworkCore5/     # EF Core 5+ support 
+├── DelegateDecompiler.Tests/                    # Core library tests
+└── DelegateDecompiler.EntityFramework*.Tests/   # EF integration tests
+```
+
+## Usage Example
+
+```csharp
+class Employee
+{
+    [Computed]
+    public string FullName => FirstName + " " + LastName;
+    public string FirstName { get; set; }
+    public string LastName { get; set; }
+}
+
+// Usage in LINQ - decompiles FullName to FirstName + " " + LastName
+var results = employees
+    .Where(e => e.FullName == "John Doe")
+    .Decompile()
+    .ToList();
+```
+
+## Development Notes
+
+- **Target Frameworks**: .NET Framework 4.0/4.5, .NET Standard 2.0/2.1, .NET 8.0/9.0
+- **Key Dependency**: Mono.Reflection for IL reading
+- **Testing**: NUnit with automatic documentation generation for EF tests
+- **Architecture**: Pluggable processor pattern for handling IL opcodes
+- **Performance**: Decompilation results are cached using `ConcurrentDictionary`