build: switch to oxfmt and oxlint

[TheGreatAxios]

Closes CL-9378

Summary

• Stacked on build: mark the package side-effect free #13, the tip of the open stack, so the reformat covers every file on it
• Replace ESLint and Prettier (added in build: add eslint and prettier with a check script #11) with oxfmt and oxlint, exact-pinned devDependencies; removes eslint, @eslint/js, typescript-eslint, jiti, prettier, eslint.config.ts, .prettierrc.json and .prettierignore
• check now runs bun run format:check instead of prettier --check .; tsconfig.json no longer includes eslint.config.ts; CONTRIBUTING describes the new tools
• CI already runs bun run check, so it picks up lint and format with no workflow change
• No lint fixes needed
• .oxfmtrc.json sets printWidth: 80 (Prettier's width); everything else is oxfmt's defaults, including sorting package.json keys
• .oxlintrc.json turns the correctness category from warn to error; no rules are disabled
• lint runs oxlint, format runs oxfmt, format:check runs oxfmt --check

Commits

1. build: switch to oxfmt and oxlint: tooling, configs and doc/script references
2. style: format with oxfmt: mechanical reformat only (1 file changed, 18 insertions(+), 18 deletions(-): package.json key order), safe to skip in review