llms.txt: meta() driven self-documentation for pieces (line + box)
Pieces can now advertise their input model via the existing meta() export
(controls / keys / params / example). The llms.txt generator (rewritten
from bash to Node) extracts these fields and surfaces them as indented
hints under each piece, so an agent can learn how to drive a piece
before loading the runtime. Covers line and box brushes as the first
two examples; remaining pieces fall back to first-comment summaries.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>