Docs: connect buyer proof to technical reference

[SamCT86]

Goal

Remove ambiguity between Agent Cash Cow's buyer-facing Transaction Lab and this technical Forecast Evidence reference.

Delta

• adds Start with the right proof surface;
• tells buyers/non-technical reviewers to start with the synthetic Transaction Lab;
• tells technical reviewers what this GitHub repo proves;
• states that neither surface proves paid adoption, forecast advantage, customer ROI or production-scale economics;
• adds a concrete portfolio/email next step.

README-only. Private production OS remains private; no runtime/deployment effect.

Base: main@21f18d50db978c6472a73852a7cf67907f065d92.