Add lefthook for local pre-commit checks

[Wikid82]

Problem

`CLAUDE.md`'s Definition of Done references lint/test/coverage gates that all run in CI, but nothing enforces them locally before a commit is made — the project name-checks "lefthook" as the intended tool (per project convention) but no `lefthook.yml` exists yet, so a broken build/lint/test can be committed and only caught at CI/PR time.

Proposal

• Add `lefthook` as a devDependency and a `lefthook.yml` at the repo root.
• Wire a `pre-commit` hook running `npm run lint` (tsc --noEmit) and, at minimum, `vitest run` scoped to changed files (or the full fast suite, given the project's small size).
• Add a `postinstall`/README step (`npx lefthook install`) so contributors get the hooks automatically after `npm install`.
• Document the hook in `CLAUDE.md`'s Definition of Done section once it exists.

Notes

Keep it fast — a slow pre-commit hook gets bypassed. Consider running the full `npm run test:coverage`/patch-report only in CI, not on every commit.