Add .editorconfig so editors match ruff and LF

## Task

CONTRIBUTING requires LF; ruff owns Python style. Editors have no shared config. Pairs with #14 (`.gitattributes`).

## Scope

Root `.editorconfig`:
- `charset = utf-8`, `end_of_line = lf`, `insert_final_newline = true`
- Python: space indent 4
- YAML/JSON/GitHub workflow: space indent 2
- Binary assets: unset

Link from CONTRIBUTING code-style section.

## Acceptance criteria

- [ ] File committed
- [ ] Matches `pyproject.toml` ruff indent
- [ ] No wholesale reformat of existing files

Start with pyproject.toml to confirm Ruff’s indentation settings, then read the code-style section in CONTRIBUTING. Add the root .editorconfig entries described in the issue and link it from CONTRIBUTING without reformatting existing files. Done means the file is committed, the settings match Ruff, and no wholesale formatting changes are present.

Written by the indexing model from the issue text.