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

[sahrizvi]

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.