4.1-nightly-2026-06-07

chore(tooling): migrate JS/TS lint and format to the oxc toolchain

Replace ESLint with oxlint and Prettier (for JS/TS) with oxfmt:

- ESLint -> oxlint: add .oxlintrc.json mirroring the previous config
  (ignore patterns, no-unused-vars ^_ exceptions, typescript/unicorn/oxc
  plugins, correctness category); remove eslint, @eslint/js,
  typescript-eslint and eslint.config.mjs.
- Prettier -> oxfmt for JS/TS: add .oxfmtrc.json (migrated from
  .prettierrc.json) and .oxfmtignore. Prettier is kept but scoped to
  JSON/YAML via .prettierignore; SCSS/CSS stay with stylelint. oxfmt uses
  its own --ignore-path so the two formatters never overlap.
- Wire oxlint + oxfmt into husky pre-commit/pre-push and CI build.yml.
- Fix two findings oxlint's default plugins surfaced: unnecessary spread
  over a NodeList (groups.ts) and a dead recursion-only parameter
  (connectSSE in chat/index.ts).
- Update AGENTS.md, docs and CHANGELOG.