observatory: restructure docs into user-friendly README + REFERENCE.md

Replaces the 983-line technical-reference README with a progressive-disclosure
document (concept -> workflow -> usage -> extension -> reference links). Moves
all contract tables, API references, JS callbacks, and performance notes into
a new REFERENCE.md. New README introduces Observatory to new users before
diving into architecture details.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: quic-boyuc