Issue 137: show runner image freshness

[rawkintrevo]

Closes #137

Summary

• Persist deployed runner image and revision metadata after Cloud Run create/restart, and compute image freshness against the current runner catalog.
• Re-resolve the selected runner image on restart so stale sessions pick up the current image, with stopped/provisioning/legacy sessions reported as unknown.
• Add separate frontend freshness indicators for latest/stale/unknown and emphasize restart for stale running sessions.
• Extend the session-create QA manifest to assert latest-image freshness after the runner is live.

Validation

• npm run check
• npm --prefix functions test
• npm run test:frontend -- sessionPresentation
• firebase deploy --only functions --project pi-agents-cloud
  • Final deployed revisions: api-00126-xud, reapIdleSessions-00114-mus
• Browser QA: e2e/qa/cases/session-create-basic.json
  • Evidence: artifacts/qa/session.create-basic/session-detail-rerun.png
  • Evidence: artifacts/qa/session.create-basic/post-delete-session-rerun-snapshot.txt
  • Initial QA found a /subagents 500; fixed handler dispatch, redeployed, and reran with no new API errors after 2026-06-25T16:20:33Z.