Add progress output for revalidate

[twidtwid]

Summary

• emit stderr progress events for clawpatch revalidate, matching review progress visibility
• include run start, per-finding start/done, aggregate done, and failure events
• cover the progress stream in the workflow revalidation test

Test plan

• corepack pnpm typecheck
• corepack pnpm test
• corepack pnpm lint
• corepack pnpm format:check

[twidtwid]

this patch eliminates confusion about whether revalidate is doing anything or not, matching how review works.