📝 Document .editorconfig as oxfmt fallback

Recommend `EditorConfig.EditorConfig` in .vscode/extensions.json so editors
without native `.editorconfig` support pick it up, and add an "Editor
configuration" section to CONTRIBUTING.md: `.editorconfig` is an
editor-compatibility fallback for the file types oxfmt does not format
(shell scripts, dotfiles, the commit-message template, git's
`COMMIT_EDITMSG` buffer), with `.oxfmtrc.json` authoritative where both
apply. `.editorconfig` itself is unchanged.

.vscode/extensions.json

@@ -3,6 +3,7 @@
         "oxc.oxc-vscode",
         "streetsidesoftware.code-spell-checker",
         "typescriptteam.native-preview",
-        "sandy081.todotasks"
+        "sandy081.todotasks",
+        "EditorConfig.EditorConfig"
     ]
 }

CONTRIBUTING.md

@@ -13,6 +13,10 @@ CI and review will bounce these even though `npm run check` and the linters don'
 - **New work starts with `npm run create:branch`, never a hand-written `git switch -c`/`git checkout -b`.** The command carries the branch precondition; branching around it skips the clean-tree, current-`main` and green-baseline checks, and the skip is invisible until a failure can no longer be attributed. (see [Branching model](#branching-model))
 - **There is no local `npm run publish`, and `publish:publint` / `publish:attw` don't go in `check`.** (see [Publishing workflow](#publishing-workflow))
 
+## Editor configuration
+
+`.editorconfig` is for editor compatibility, not a gate — it is a sane fallback for the files oxfmt does not format (shell scripts, dotfiles, `LICENSE`, the commit-message template, and git's `COMMIT_EDITMSG` buffer). Where both apply, `.oxfmtrc.json` is authoritative: oxfmt is the formatter, and the overlapping `.editorconfig` keys only keep non-oxfmt editors close to the formatted result.
+
 ## Commit messages
 
 Gitmoji subject, imperative mood, 50/72 wrapping. The template is `commit-message-template`; run `npm run setup:git-commit-message` once after cloning to register it as git's `commit.template` (or `npm run setup` to run every one-time clone step).