Make registry the single visible plugin distribution path

[ascorbic]

Goal

1.0 should have one plugin catalog, one install flow, one capability-consent model, and one trust story.

Scope

Ensure the registry is the visible product path for plugin discovery and installation, and remove or hide competing legacy distribution surfaces from the user-facing product.

Acceptance criteria

• Admin users see one plugin discovery/install path.
• Documentation points to the registry path.
• Legacy marketplace surfaces are removed, hidden, or clearly deprecated.
• Shared lower-level bundle/storage utilities remain only where needed by the registry path.
• Tests cover the registry browse/install path that remains.