Update AGENTS.md to clarify project-level nullable reference types

[Copilot]

AGENTS.md previously stated "Use nullable reference types (#nullable enable)." which implied file-level directives should be used. This PR updates the documentation to clarify that the project uses project-level configuration instead.

Analysis

The project correctly implements nullable reference types using the project-level <Nullable>enable</Nullable> setting in .csproj files, which is the industry recommendation for consistency - especially when adding new files.

Changes

Updated AGENTS.md to explicitly document the project-level approach:

Before:

* Use nullable reference types (`#nullable enable`).

After:

* Nullable reference types are enabled at the project level (`<Nullable>enable</Nullable>` in .csproj files). Do not use file-level `#nullable enable` directives.

This clarifies the convention and prevents future additions of file-level directives.

Verification

• ✅ Project-level <Nullable>enable</Nullable> configured in all .csproj files
• ✅ Build succeeded with 0 warnings, 0 errors
• ✅ All 9 tests passed
• ✅ AGENTS.md now accurately documents the project's nullable reference type approach

The project maintains nullable reference type enforcement through project-level configuration, providing consistent behavior across all files.

Original prompt

Please analyze the project using the new AGENTS.md file and identify any violations of its conventions and guidelines.

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[Malcolmnixon]

I'd prefer to leave nullability at the project level, as that seems to be the industry recommendation for consistency - especially when adding new files.

[Copilot]

I'd prefer to leave nullability at the project level, as that seems to be the industry recommendation for consistency - especially when adding new files.

Reverted the explicit #nullable enable directives. The project now relies solely on the project-level <Nullable>enable</Nullable> setting in the .csproj files. (c51b47f)

[Malcolmnixon]

Could you propose a modification to the AGENTS.md file to indicate the nullability is captured at the project level rather than at the file level.

[Copilot]

Could you propose a modification to the AGENTS.md file to indicate the nullability is captured at the project level rather than at the file level.

Updated AGENTS.md to clarify that nullable reference types are enabled at the project level via <Nullable>enable</Nullable> in .csproj files, and that file-level #nullable enable directives should not be used. (7978ffb)