Move GOAL.md, PRODUCT.md, SPEC.md from repo root into doc/. Add AGENTS.md (contributor guidance), doc/DEVELOPING.md (dev setup), doc/SPEC-implementation.md (V1 implementation contract), and doc/specs/ui.md (UI design spec). Update ClipHub doc. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>