build: switch to oxfmt and oxlint

[TheGreatAxios]

Closes CL-9371

Summary

• Remove ESLint (eslint, @eslint/js, typescript-eslint, jiti, eslint.config.ts) and Prettier (prettier, .prettierrc.json)
• Add oxfmt and oxlint as exact-pinned devDependencies
• CI runs bun run format:check in place of bunx prettier --check .; the existing bun run lint step now runs oxlint
• IMPLEMENTATION.md: dev dependency list names oxlint and oxfmt instead of ESLint and Prettier
• .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

Stacked on #14. This supersedes the ESLint setup from #12, which stays open below it in the stack.

Commits

1. build: switch to oxfmt and oxlint: tooling, configs, CI and docs
2. style: format with oxfmt: mechanical reformat only (1 file changed, 21 insertions(+), 21 deletions(-)), safe to skip in review