fix: preserve review UX across new pushes

[neversettle17-101]

Summary

• keep the latest completed AO review verdict available as previousRun when a PR head advances
• show the previous verdict in the reviewer UI while the new-head review is running
• auto-trigger AO review when SCM observes a new PR head for sessions that already have reviews enabled

Fixes #2300

Tests

• go test ./internal/review ./internal/lifecycle ./internal/httpd/...
• npm run frontend:typecheck