chore(brand): add impeccable PRODUCT.md + design-tooling glue
Seed impeccable's strategic layer via `/impeccable init`, register `brand`,
distilled from VISION.md + BRAND.md (not invented). PRODUCT.md is a new tracked
doc and passes remark --frail.
Gitignore the root DESIGN.md discovery symlink → brand/DESIGN.md: it is
local-only tooling glue so `/impeccable` resolves the design system. Committing
it breaks check:md — remark-validate-links follows the symlink and resolves
brand/'s relative links (./tokens.css, ../BRAND.md, ./HANDOFF.md) from the repo
root, failing under --frail. brand/DESIGN.md stays the one tracked, linted copy;
no lint coverage is lost.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>