[Repo Assist] eng: add .editorconfig to enforce consistent C# code style

[github-actions]

🤖 This is an automated draft PR from Repo Assist.

Summary

Adds a root-level .editorconfig to the repository. There was no editor config previously — this captures the conventions already in use so that IDEs and dotnet format agree with the existing codebase.

What it covers

Area	Setting
Encoding / line endings	UTF-8, CRLF (Windows project)
Indentation	4-space for C#; 2-space for YAML, JSON, XML
Brace style	Allman (csharp_new_line_before_open_brace = all) — matches current code
Namespace declarations	file_scoped — matches codebase convention
var preference	var when type is apparent, explicit otherwise
Null / pattern matching	prefer is null, ?., ??
Private field naming	_camelCase — matches current code
Readonly	prefer readonly fields where possible
Trailing whitespace / final newline	trimmed / enforced

The .editorconfig is advisory (:suggestion/:silent severities) — no existing code will be flagged as errors. It gives contributors consistent formatting from their first keystroke.

Why

• Without a .editorconfig, VS Code, Visual Studio, and Rider all default to different formatting choices (tabs vs spaces, namespace styles, etc.).
• Once in place, dotnet format --verify-no-changes can be added to CI as a future zero-cost linting gate.

Test Status

✅ All 503 shared tests pass, 18 skipped. dotnet test tests/OpenClaw.Shared.Tests/
⚠️ WinUI build requires Windows — no functional code was changed; .editorconfig is tooling metadata only.

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