Add Directory.Build.props with shared build settings

[LeeCampbell]

Summary

Add a Directory.Build.props file at the repository root to centralise shared MSBuild properties across all projects.

Details

This file should include:

• EnforceCodeStyleInBuild: Set to true so .editorconfig style rules are checked during build
• AnalysisLevel: Set to latest-recommended to enable the built-in .NET analysers
• Nullable: Enable nullable reference types repo-wide (if not already set per-project)
• TreatWarningsAsErrors: Consider enabling for analyser warnings (CA/IDE rules) — but assess the impact on the current codebase first
• ImplicitUsings: Align on whether this is enabled or disabled repo-wide

The goal is to have one place for build-wide settings rather than duplicating them in each .csproj.

Acceptance Criteria

• Directory.Build.props added at repository root
• EnforceCodeStyleInBuild enabled
• Built-in .NET analysers enabled at recommended level
• Build and tests still pass (adjust severity levels if needed to avoid breaking the build)
• Any per-project settings that are now redundant are removed from individual .csproj files

Notes

Start conservative — if enabling analysers produces too many warnings, set the severity to suggestion initially and tighten in follow-up issues.

[leecampbell-codeagent]

PR created: #127