chore: add Prettier for consistent code formatting

Note: this is an in-house item already being handled by the maintainer - not open for contribution. Filed for tracking only.

Add Prettier as a dev dependency to enforce consistent code formatting across all source files.

Blocked on: #443 (adds braces to single-line if statements in src/cli/args.ts) — merge that first to avoid conflicts.

Work:

• Install prettier as a dev dependency
• Add .prettierrc with project style rules (single quotes, 2-space indent, trailing commas, printWidth 120, arrowParens always)
• Run Prettier on the full codebase in a single formatting commit
• Add format and format:check scripts to package.json
• Add a format check step to .github/workflows/ci.yml
• Update CONTRIBUTING.md to document the npm run format workflow

Prettier config:

{
  "semi": true,
  "singleQuote": true,
  "tabWidth": 2,
  "trailingComma": "all",
  "printWidth": 120,
  "arrowParens": "always"
}