docs: README — lead with an agent-interaction screenshot (#32)

Add a hero screenshot near the top of the README showing the sensorwatch
agent monitor answering "how's the PSU efficiency been today?" in plain
English — a scoped report of the PSU's efficiency, power, and temperature
over the day. It highlights the natural-language, agent-driven way to check
in on hardware without parsing raw logs, complementing the existing
"flagship result" prose just below it.

- docs/images/agent-monitor-psu-query.png: the screenshot asset (new
  docs/images/ directory).
- README.md: embed it as a clickable (full-size) image with a caption
  pointing at the agent monitor skill.

Co-authored-by: lcj-claude-coder <lcj.claude.coder@pm.me>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>