feat(desktop): add independent eval verdict gate

[gagan114662]

Summary

• add a conservative Eval Agent verdict gate
• require versioned, pre-locked owner criteria and an evaluator independent from the evaluated agent
• require both external outcome evidence and process/trace evidence
• block promotion without a validated verdict, non-regressing baseline comparison, and owner approval

Verification

• full Desktop unit suite passed
• Desktop TypeScript typecheck passed
• Desktop repository checker passed; it reports two pre-existing template-literal infos and one pre-existing CSS warning

Scope

This is the smallest foundational slice for the Eval Agent. Persistence, repeated-run orchestration, and owner-facing UI follow in later slices.