Add .editorconfig

[nora-weisser]

Goal

Add an .editorconfig for consistent formatting across editors, matching Prettier.

Scope

Create .editorconfig at repo root:

• indent_style = tab
• end_of_line = lf
• charset = utf-8
• insert_final_newline = true
• trim_trailing_whitespace = true (relax for *.md)

Acceptance criteria

• .editorconfig exists and matches Prettier config
• npm run format reports no changes on a freshly-edited file