Add a shared package

First check the existing packages (packages/*/package.json descriptions): a helper that fits one of them goes there. Code for one service stays in that service.

1. bun run gen:new package --args <name> "<one sentence: what it is for>", e.g. bun run gen:new package --args money "Formatting and arithmetic for amounts of money." It writes packages/<name>/ (package.json: private, type: module, exports ./src/index.ts, check-types/test/coverage scripts, @repo/typescript-config and @repo/vitest-config), formats, and runs bun install to link it. <name> is a commit scope at once (commitlint.config.ts reads the workspace folders).
2. Write the code in src/, with a *.test.ts beside each file. Every file is at 100% coverage (bun run test:coverage; docs/testing.md, "Coverage").
3. Dependencies: "catalog:" when the root package.json catalog has the package, otherwise a normal range. Node types: add @types/node ("catalog:") and "compilerOptions": { "types": ["node"] } to its tsconfig.json, like packages/logger. React code extends @repo/typescript-config/react-library.json instead.
4. Consumers add "@repo/<name>": "workspace:*" to their package.json, then bun install. Apps and mobile must not import server-only packages; if the new one is server-only, add it to the web-has-no-backend-code rule in .dependency-cruiser.cjs.
5. bun run lint (knip flags unused exports and undeclared dependencies), bun run check-types, bun run test.
6. The reviewer agent on the change.