Porcelain gc - invoke garbage collection

[mroczect]

Summary

Implement gc command that calls plumbing gc to remove unreachable objects.

Context

May include additional optimizations like compressing object store.

Tasks

• Create porcelain/src/admin/gc.rs.
• Call plumbing gc.
• Report statistics (e.g., objects removed).
• Document.

Acceptance Criteria

• Unreachable objects removed.

Dependencies

• Plumbing gc.