Add ESLint across monorepo packages #5

Summary

pnpm lint currently runs no tasks. Add ESLint (or equivalent) to each package and wire it through turbo.

Why

Contributors lack automated style feedback. CI only runs build + test.

Suggested approach

1. Add shared ESLint config at repo root
2. Add "lint": "eslint src" to each packages/*/package.json
3. Optionally add pnpm lint to CI workflow

Acceptance criteria

• pnpm lint executes in all 15 packages
• CI runs lint on PRs (optional but preferred)
• Document lint commands in CONTRIBUTING.md