Build: add Biome to npm check

[marcuscastelo]

Task Template

Title:
Build: add Biome to npm check

Description:
Add Biome to the repository's npm check pipeline so CI and local development run Biome linting alongside existing checks (ESLint, type-check, tests). This should include installing Biome as a dev dependency and updating the relevant npm scripts (for example flint or check) so that pnpm run check invokes Biome and fails the build on lint errors.

Motivation:
Biome is a fast, modern lint/format/static analysis tool that can complement or replace parts of ESLint/Prettier. Integrating Biome into the check pipeline ensures consistent linting is enforced in CI and by local developers, improving code quality and reducing formatting/lint-related PR churn.

Acceptance Criteria:

• Biome is added as a devDependency in package.json (document package name and version).
• pnpm run check (or the flint script) runs Biome and fails on lint errors.
• CI (pnpm run copilot:check / pnpm run check) passes with Biome enabled.
• Developer documentation (README / CONTRIBUTING) updated with instructions to run Biome locally.
• If Biome requires config, include a committed config file (e.g. .biomerc/biome.config.*) with rationale for key rules.

Additional Context:

• Current package.json scripts of interest: check -> run-p flint type-check test, and flint -> pnpm run fix && pnpm run lint (see package.json root).
• App version (from .scripts/semver.sh): $(.scripts/semver.sh 2>/dev/null || echo "unknown")