docs: Add production execution design (openadapt-agent proposal) (#977)

- Literature review: UFO, Claude CU, OSWorld, ST-WebAgentBench
- Gap analysis: safety exists but not wired to execution loop
- Recommendation: Create openadapt-agent package for production automation
- README improvement proposal: Condense from 443 to ~100 lines
- Implementation roadmap: Q1-Q3 2026

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: abrichr