Add repo-wide engineering standards (.editorconfig, SDK pinning, Directory.Build.props)

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Problem

The repo lacks common .NET engineering standards files:

• No .editorconfig for consistent formatting
• No global.json to pin the .NET SDK version
• No Directory.Build.props for shared MSBuild settings (nullable, analyzers, package versions)

This can lead to inconsistent formatting and build differences across machines/CI.

Proposed solution

1. .editorconfig — C# formatting, indent, newline rules (use dotnet defaults as baseline)
2. global.json — pin SDK to 8.0.x (match CI dotnet-version: 8.0.x)
3. Directory.Build.props — centralize:
   • LangVersion
   • Nullable (coordinate with Enable nullable reference types #29)
   • TreatWarningsAsErrors (optional, phased)
   • Shared analyzer package references (optional)

Acceptance criteria

• .editorconfig committed at repo root
• global.json pins SDK version used in CI
• Directory.Build.props reduces duplication between csproj files
• CI and local builds use the same SDK constraint