feat(registry): support scoped pack names (@scope/pack-name)

[breferrari]

Summary

Add namespace scoping to pack names using @scope/pack-name format (similar to npm's scoped packages) to prevent name collisions as the registry grows.

Motivation

The registry currently uses a flat namespace. As more packs are published, name collisions become likely (e.g., multiple teams wanting a pack called python or docker). Scoping prevents this and also provides a trust signal — @anthropic/claude-tools carries more weight than just claude-tools.

Design considerations

• The @ prefix is already used for registry installs (weave install @webdev), so the syntax would need to distinguish between @pack-name (unscoped registry) and @scope/pack-name (scoped)
• Alternative: use weave install webdev for unscoped and weave install @myteam/webdev for scoped
• Scopes could map to GitHub organizations for verification
• This should be designed before the registry has many consumers, as it's a protocol-level change

Impact

This is a breaking change to the registry protocol if added later. Better to design the namespace format now, even if scopes are optional initially.