120-line brand guidelines document covering the v0.42.0 brand
system: naming ('Bill Tracker' with the space, not 'BillTracker'),
asset paths, lockup usage rules (light vs dark variants, mono
mark, social preview), the canonical source-of-truth in
client/lib/brand.ts, and the GitHub-Open-Graph card dimensions.
This is commit 4 (and the last) of the v0.42.0 follow-up batch.
Docs-only; no code change. Depends on 68e57cf (favicon + PWA).