Focused design snapshot for the Pack/Registry CLI v2 work.

• design/pack-cli-unification.md
  • the current design note for the gc pack and gc registry surface
• cli2/
  • a mock pack that exposes the proposed command tree through help text only

This repo is intentionally small. It contains only the design doc, the mock pack, and this top-level README so the CLI proposal can be reviewed in isolation.