docs: make pre-commit hooks a mandatory constitution rule (#134)

## What

Adds **constitution rule 5 — Pre-commit hooks are mandatory** to
`CLAUDE.md`, and condenses the `Before You Start` section.

## Why

This is an open-source repo and external contributors work on it with
Claude and other AI agents. `CLAUDE.md` is what those agents read, and
it only mentioned pre-commit as setup guidance plus one clause inside
the code-quality rule. Nothing forbade bypassing the hooks — so an agent
could skip the checks on a commit and still read the file as satisfied.

The new rule sits inside the non-overridable constitution, so it
inherits MUST-force:

- Installed hooks are a **prerequisite** for working in the repo — and
if `pre-commit`, or a tool its hooks need, is missing on the
contributor's machine, it must be installed before work starts.
- Hooks must **never** be bypassed. A failing hook means fix the code,
not skip the check.

## Size

`CLAUDE.md` is loaded into every agent's context on every session, so
its length is a real cost. The setup section was condensed to pay for
the new rule: **net -8 lines** (9 added, 17 removed).