Split out of docs/marketplace.md so the design document stays about how it works.
Ranked by what blocks something rather than by size: submissions first (until a
stranger's manifest can be read without executing it, the catalogue is ours only
and everything below is cosmetic), then plugin upgrades not migrating, then
staleness, then scale.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
