[docs] add modular pipeline conventions and gotchas
Create .ai/modular.md as a shared reference for modular pipeline
conventions, patterns, and common mistakes — parallel to the existing
models.md for model conventions.
Consolidates content from the former modular-conversion.md skill file
and adds gotchas identified from reviewing recent modular pipeline PRs
(LTX #13378, SD3 #13324).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Author
yiyi@huggingface.co
