score: add "Adding a Piece" section — template, meta() visibility, curation
Documents the repo-level piece workflow that was implicit: npm run new,
header comment convention the docs auto-scan reads, meta() export as the
opt-in signal for list/autocomplete, and the docs.js pieces map as the
curation override. Points out to WRITE-A-PIECE.md (user flow) and
CLAUDE.md (API surface) for depth.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>