docs: align CONTRIBUTING and setup guide with the pre-commit rule (#136)

## What

Brings `CONTRIBUTING.md` and `docs/development/setup.md` in line with
constitution §5 (added in #134).

## Why

§5 made pre-commit hooks a mandatory, never-bypassed prerequisite — but
`CONTRIBUTING.md` did not mention pre-commit a single time. That is the
file GitHub surfaces to external contributors when they open a pull
request, and its "Before Submitting" checklist asked only for `bun test`
and `bun run type-check`. The doc aimed at humans had become the weakest
of the three.

## Changes

**CONTRIBUTING.md**
- `pre-commit` listed as a prerequisite next to Bun, with install
commands
- `pre-commit install` added to the setup step
- New short **Pre-commit Hooks (required)** section: what the hooks
enforce, install missing tooling rather than work around it, never
bypass, and `pre-commit run --all-files`
- Checklist item: hooks installed and passing, no bypassed commits in
the PR
- AI-agent section now lists hooks as a requirement and points at
`AGENTS.md` alongside `CLAUDE.md`, naming the constitution as binding

**docs/development/setup.md**
- `pre-commit` marked **required** in prerequisites
- "do not skip this" replaced with a pointer to constitution §5, plus
the rule that missing hook tooling gets installed, not worked around
- The no-bypassing rule stated directly under the hook table — where
anyone hunting for a way around a failing hook would land

Docs only; no code paths touched.