ARCHITECTURE, PRODUCT and TODO move out of the repository root, and
docs/superpowers/{specs,plans} flattens to docs/{specs,plans}:
"superpowers" names the tool that wrote those documents, not a kind of
document.
No content changes. This commit is the move, plus the roughly thirty
cross-references the move invalidated.
Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
