chore(docs): update branding image (librarian-scholar)

Replace the hero branding image with the new Gemini-generated one, scaled to
1200x654 (matching the previous dimensions; aspect ratios were identical, so no
distortion). README renders it at width=600, unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: nickroci