fix: migrate tooling from eslint/prettier/yarn/husky to biome/pnpm/lefthook

[jrandolf]

Summary

• Replace ESLint + Prettier with Biome (via ultracite) for unified linting and formatting
• Switch package manager from Yarn to pnpm (adds pnpm-lock.yaml, removes yarn.lock)
• Replace Husky git hooks with Lefthook
• Update dev dependencies to latest versions
• Reformat all source files to match Biome style (double quotes, import type usage)
• Update license from MIT to Apache-2.0
• Update GitHub CI/CD workflows (rename .yaml → .yml, add release and release-notes workflows)
• Add .node-version file for Node version pinning

Test plan

• Verify pnpm lint passes with Biome
• Verify pnpm format runs ultracite fix correctly
• Verify pnpm test passes
• Verify CI workflows trigger correctly on push/PR
• Verify Lefthook pre-commit hook runs on commit