Reframe README around Developer Brain identity

Restore the visual richness of the pre-split README (hero, badges,
LongMemEval table, ASCII diagram, comparison table, FAQ) and merge in
the post-split positioning: git-shared repo context as a first-class
feature and a Public vs Enterprise table that hands off to dhee-enterprise.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: Ashish-dwi99