#55 docs(readme): introduce agent-led calculation workflow

Summary

Replace the stack-first introduction with an agent-led calculation workflow for readers using Excel, calculation software or Python. Centre the title, subtitle and opening paragraphs above the source-to-document screenshot.

- Stack explanation and prototype warning lead the README
+ Agent workflow, screenshot and worked-example link
+ Source-to-document safeguards, reusable calculations and scientific libraries
+ Starter prompt, setup and concise project status

Related to #54 for the scientific-library capability described in the README. This PR changes documentation and adds the screenshot; it does not implement imported calculation functions.

Evidence

• Before: The README opened with technical constraints and stack choices, without a worked-example image.
  After: Four centred introductory elements precede the linked screenshot and live example.
• Checked all 23 local links, the setup anchor and the PNG asset; the Python excerpt matches the canonical example.
• The documented verification command passed: five inputs, seven formulas and seven independent reference values. PDF rendering and visual inspection were not rerun for this documentation change.
• git diff --check and the staged whitespace check passed.

Merge Danger

Door: two-way

Revert the README and image to restore the previous presentation.

Blast Radius: documentation