chore: remove stale campus-era design docs

design-system/ (MASTER, editorial-minimalist, neobrutalist-bento) and
upgrade-summary.md were pre-street-edition design logs — campus framing,
old "Monsoon Paper" palette, tokens the shipped design no longer uses.
Not referenced by any code. Current design direction lives in the app
itself + docs/. History preserves these if needed.

Author: thribhuvan003