Document strict acceptance proof requirements

[ericmichael]

Summary

• Add explicit acceptance proof requirements for launcher UI, agent, chat, Routine, autopilot, and background-run work.
• Make clear that intermediate states like toasts, banners, opened tabs, or lifecycle badges are not sufficient proof.
• Add a PR proof checklist requiring final user-visible outcomes and transcript/session-history evidence for agent/model features.

Validation

• Docs-only change; no runtime tests run.