Add explicit build and test requirements to instructions

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

@@ -2,6 +2,19 @@
 
 This repository contains the official C# SDK for the Model Context Protocol (MCP), enabling .NET applications to implement and interact with MCP clients and servers.
 
+## Critical: Always Build and Test
+
+**ALWAYS build and run tests before declaring any task complete or making a pull request.**
+
+When making code changes:
+1. **Build first**: Run `dotnet build` or `make build` to ensure the code compiles without errors
+2. **Run tests**: Execute `dotnet test --filter '(Execution!=Manual)'` or `make test` to verify all tests pass
+3. **Fix issues**: Address any build errors or test failures before proceeding
+4. **Verify iteratively**: Build and test frequently during development, not just at the end
+5. **Check warnings**: Treat warnings as errors - the build is configured with `TreatWarningsAsErrors=true`
+
+**Never skip these steps.** Even small changes can have unexpected impacts. A passing build and test suite is the minimum bar for any code change.
+
 ## Project Overview
 
 The SDK consists of three main packages:
@@ -97,6 +110,16 @@ The SDK consists of three main packages:
 - **Test**: `dotnet test` or `make test`
 - **Clean**: `dotnet clean` or `make clean`
 
+### Development Workflow
+**Critical**: Always follow this workflow when making changes:
+1. Make code changes
+2. Build immediately: `dotnet build` - fix any compilation errors
+3. Run tests: `dotnet test --filter '(Execution!=Manual)'` - fix any test failures
+4. Repeat steps 1-3 iteratively as you develop
+5. Only after successful build and tests should you consider the change complete
+
+Do not skip or defer building and testing. These are mandatory steps for every code change, no matter how small.
+
 ### SDK Requirements
 - The repo currently requires the .NET SDK 10.0 to build and run tests.
 - Target frameworks: .NET 10.0, .NET 9.0, .NET 8.0, .NET Standard 2.0