chore: add pnpm + oxfmt for repo formatting

Pins Node 24 via .node-version, declares pnpm 10.32.1 as packageManager
in package.json, and adds oxfmt as a dev dependency with .oxfmtrc.json
(single quotes, no semi, 80 cols). Ignores node_modules.

.oxfmtrc.json

@@ -0,0 +1,8 @@
+{
+  "$schema": "./node_modules/oxfmt/configuration_schema.json",
+  "singleQuote": true,
+  "semi": false,
+  "printWidth": 80,
+  "sortPackageJson": false,
+  "ignorePatterns": []
+}