Add an AGENTS.md file.

Author: Malcolm Nixon

AGENTS.md

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+# AI Instructions for SpdxModel
+
+This file provides specific context and instructions for AI coding agents to
+interact effectively with this C# project.
+
+
+## Project Overview
+
+SpdxModel is a C# library for serializing and deserializing SPDX SBOMs into an
+in-memory representation suitable for manipulation and analysis.
+
+
+## Technologies and Dependencies
+
+* **Language**: C# 12
+* **.NET Frameworks**: .NET 8, 9, and 10
+* **Primary Dependencies**: [System.Text.Json]
+
+
+## Project Structure
+
+The repository is organized as follows:
+
+* `/.config/`: Contains the .NET Tool configuration.
+* `/.github/workflows/`: Contains the CI/CD pipeline configurations.
+* `/src/DemaConsulting.SpdxModel/`: Contains the library source code.
+* `/test/DemaConsulting.SpdxModel.Tests/`: Contains the library unit tests.
+* `/DemaConsulting.SpdxModel.sln`: The main Visual Studio solution file.
+
+
+## Development Commands
+
+Use these commands to perform common development tasks:
+
+* **Restore DotNet Tools**:
+  ```bash
+  dotnet tool restore
+  ```
+
+* **Build the Project**:
+  ```bash
+  dotnet build
+  ```
+
+* **Run All Tests**:
+  ```bash
+  dotnet test
+  ```
+
+
+## Testing Guidelines
+
+* Tests are located under the `/test/DemaConsulting.SpdxModel.Tests/` folder and use the MSTest framework.
+* Test files should end with `.cs` and adhere to the naming convention `[Component]Tests.cs`.
+* All new features should be tested with comprehensive unit tests.
+* The build must pass all tests and static analysis warnings before merging.
+* Tests should be written using the AAA (Arrange, Act, Assert) pattern.
+
+
+## Code Style and Conventions
+
+* Follow standard C# naming conventions (PascalCase for classes/methods/properties, camelCase for local variables).
+* Use nullable reference types (`#nullable enable`).
+* Warnings are treated as errors (`<WarningsAsErrors>true</WarningsAsErrors>`).
+* Avoid public fields; prefer properties.
+
+
+## Boundaries and Guardrails
+
+* **NEVER** modify files within the `/obj/` or `/bin/` directories.
+* **NEVER** commit secrets, API keys, or sensitive configuration data.
+* **ASK FIRST** before making significant architectural changes to the core library logic.