docs: correct the README's stale claims and document the staging workflow

[LKSNDRTMLKV]

Stacked on #19. The README describes a repository that no longer exists in several places, and one of those descriptions asserts a capability the tool has never had.

The claim that mattered

# Type-check + broken-link check
pnpm -r check

It is not a broken-link check and never was. A markdown link target is an opaque string to astro check, which is precisely why four [Licensing](/engine/licensing) links passed it and 404'd in production. A contributor reading this would reasonably believe links were covered, and stop looking — the same trap as a comment that describes a guard the code does not implement.

Corrected, and the four gates CI actually runs are now listed with what each one does.

Stale after recent work

• The layout diagram listed a workspace-root public/brand/ directory that no longer exists — its copy step was publishing a duplicate favicon at a path nothing referenced — and omitted scripts/, which now holds two of the gates.
• The Status section described CI as gating build + check. It runs six steps.
• It pointed at the docs site under Design Principles, a section that was removed; the link survived only because a redirect happened to catch it. Now points at the pages that exist.
• "What remains before public launch" listed the a11y pass and deployment, and predated the audit entirely.
• The deploy explanation described pushing to main, which the new ruleset forbids.

Adds what was undocumented

staging exists and main is protected, and nothing in the repository said so. A contributor's first instinct is to branch off main and open a pull request against it — which now fails at the ruleset rather than at review. There is a short section describing the flow: work branches off staging, promotion is a separate staging → main pull request, and main requires a pull request, squash-only merges, passing CI, and permits no bypass for anyone.

The Status section also now records what the audit found, in a sentence rather than a list, without pointing at the register — that lives in a private repository and public artefacts stay self-contained.

Verification: build green · 828 links resolve · leakage clean.

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