Add CONTRIBUTING.md with development setup and guidelines

[LeeCampbell]

Summary

Add a CONTRIBUTING.md file to help new contributors (human and agentic) get started quickly and understand the project's expectations.

Details

The file should cover:

• Prerequisites: .NET SDK version, supported platforms
• Building: How to build the solution
• Testing: How to run tests
• Code style: Reference to .editorconfig, how to run dotnet format locally before submitting a PR
• Git workflow: Branch naming conventions (feat/, fix/, chore/), PR expectations, targeting main
• Line endings: Explain the LF convention and .gitattributes setup
• Cross-platform notes: Any gotchas for Windows/macOS/Linux contributors

Acceptance Criteria

• CONTRIBUTING.md added at repository root
• Covers build, test, format, and PR workflow
• Uses British English, one sentence per line (per project markdown standards)
• Referenced from the main README.md if appropriate

[leecampbell-codeagent]

PR created: #129