docs: publish rule catalog for check --checks safety (and lint / policy)

Deferred from the **v0.9.6 release review** (End User + PM personas).

## Context

v0.9.6 adds a new safety rule from the engine: `unbalanced_quote`. Users only learn its name when it fires — there's no rule catalog in the docs. Same for `lint` (mentions L003 in the example only) and `policy` (rules are user-defined but there's no reference for the built-ins).

The v0.9.6 release patch added a one-line hint in `docs/docs/usage/check.md`:
> Every finding carries a machine-readable `rule` field... visible in `--format json` output, and how the safety, lint, and policy rule inventory is discovered in practice.

That's a workaround, not a catalog.

## What to do

- Add a "Safety Rules" section to `docs/docs/usage/check.md` listing every rule the engine can emit (name + one-line description + example)
- Same for `lint` (built-in rules) and `policy` (built-in policy checks)
- Auto-generate from engine metadata if the engine exposes it — otherwise curated markdown, updated with each altimate-core bump
- Cross-link from the Available Checks table so users can navigate: check → rule catalog → example

## Priority

P2 — no user is blocked, but "what's `unbalanced_quote`?" is a certain support-ticket generator on Monday morning after upgrade.