Adds the customer-facing documentation for the manifestos + brainstormer
feature that closed the cosmetic-tickets gap. Real customers consuming
this OSS need to know:

1. The four files they own (.forge/product-vision.md, axes.yaml,
   quality-manifesto.md, testing-manifesto.md).
2. The brainstormer dry-run + --apply workflow.
3. The feedback loop (`forge-loop manifesto suggest --from-pr <N>`)
   where every bug becomes a permanent gate.
4. What the worker + critic see (manifestos injected into briefs;
   sev1 violations block auto-merge).

README: new section "Manifestos & the brainstormer (axis-aligned
tickets)" between Briefs and CLI reference. CLI reference table gains
`brainstorm`, `brainstorm --apply`, `manifesto suggest --from-pr`.

GUIDE: new section 4 "Manifestos: drive what gets built (not just how)"
between "discipline matters" and "the brief is your contract" — with
the real Titan brainstormer output as the worked example. Sections
5-10 renumbered accordingly.

Both docs cite PR #147 as the canonical feedback-loop example: a
stringly-typed event-boundary bug that surfaced after #97/#120/#128
all had the same shape; the fix landed the manifesto rule that the
critic now enforces.