Build the read-only review screen and complete the go/no-go gate #3

After runner/store is ready, build approved step 2 with DESIGN.md tokens and the linking library: per-item segments, four status checks, file-change cards, assignments, approval/staleness, and error/empty states. No merge or agent execution controls. Complete the manual real-issue assignment, commit the experiment protocol and ledger-aware plant script before reviews, then run the documented paired real-PR go/no-go test. Record results before proceeding to steps 4–9. Include GitHub already-fixed self-PR exclusion in its later runner/github integration, not the pure linking engine.