Add .editorconfig with project coding conventions

[LeeCampbell]

Summary

Add an .editorconfig file to the repository root to codify the project's coding conventions.
This is the foundation for all subsequent code style enforcement.

Details

The .editorconfig file should capture the conventions already present in the codebase (not impose new ones).
Key settings to include:

• General: root = true, charset utf-8, LF line endings, final newline, trim trailing whitespace
• Indentation: 4 spaces for C#, appropriate settings for XML/JSON/YAML
• C# code style: namespace declaration style, var usage preferences, brace placement, using directive placement
• C# naming conventions: PascalCase for public members, _camelCase for private fields (match existing code)
• Severity levels: Start with suggestion or warning — not error — so this PR doesn't break the build

Acceptance Criteria

• .editorconfig file added at repository root
• Settings reflect the conventions already used in the existing codebase
• dotnet build still succeeds without new errors
• Settings documented with comments where the convention might not be obvious

Notes

This issue is a prerequisite for subsequent code style enforcement work.
Do NOT run dotnet format to reformat existing code — that is a separate issue.

[leecampbell-codeagent]

PR created: #124