Design first, build after approval

Draft, review, redo, and approve Arka implementation plans before code changes.

arka design plan "build a dashboard for local model benchmarks"
arka design show
arka design redo "include offline support and a CSV export"
arka design accept
arka design build --yes

design plan and design redo do not modify the project. design build is blocked until the draft is accepted, keeping planning separate from execution.