Add Husky pre-commit hooks with lint-staged

[PAMulligan]

Summary

Standardize formatting and linting across widget/ and worker/ with shared config, and block bad commits at the local level via Husky + lint-staged.

Motivation

The project is already large enough that inconsistent style and lint drift will create review friction. Locking this in now is much cheaper than retrofitting later.

Acceptance Criteria

• Root eslint.config.js (flat config) with shared base + per-package overrides for widget (React) and worker (Cloudflare)
• prettier configured with .editorconfig and a .prettierignore
• Husky + lint-staged: eslint --fix and prettier --write on staged files
• commitlint enforcing Conventional Commits (matches existing branch-prefix convention)
• CI job runs pnpm lint and pnpm format:check and fails on violations
• CONTRIBUTING.md updated with the new toolchain

[PAMulligan]

Rescoped to the remaining work: ESLint and Prettier already exist in widget/ (lint, format, format:check scripts) and CI enforces both plus typecheck on every push/PR (ci.yml). What is left from the original scope is the local pre-commit guard: Husky + lint-staged running eslint --fix and prettier --write on staged files, with a note in CONTRIBUTING.md. Watch out for the Windows CRLF/autocrlf interaction when wiring prettier into hooks.