chore: add .editorconfig with standard defaults

[tayloralessia207]

Adds a top-level .editorconfig so contributors on different editors get consistent indentation, line endings, and trailing-whitespace handling out of the box.

No effect on existing files — EditorConfig is only applied by editors that support it (which is most of them these days).

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[gemini-code-assist]

Code Review

This pull request introduces an .editorconfig file to establish consistent code formatting rules across the project. The review feedback suggests setting the indentation size for Markdown files to 2 spaces to align with Prettier's configuration and avoid editor conflicts.

[gemini-code-assist]

Since the project uses Prettier to format Markdown files (as configured in the lint:other script in package.json), which defaults to a 2-space indentation, leaving *.md to inherit the default indent_size = 4 from [*] will cause a conflict between the contributor's editor and the formatting checks. Specifying indent_size = 2 for *.md ensures consistency with Prettier.

[*.md]
indent_size = 2
trim_trailing_whitespace = false