Rewrite README intro with second-opinion framing

[eph5xx]

Summary

• The old intro led with mechanics ("14 weighted dimensions, evidence grades, assumption tracking") which describes the tool but doesn't tell a first-time visitor why they'd care. The new intro frames TweakIdea as an independent second opinion — free, evidence-backed, 10 minutes — and answers the obvious "why not just ask ChatGPT?" question right at the top.
• Replaced the lead paragraph with a concise positioning line, a "What you get" bullet list, and a "Why not just ask ChatGPT?" comparison.
• Promoted the example reports + screenshot above the fold (previously buried below the transcript) so visitors can see the output before committing to read further.

Type of change

• Documentation

Test plan

• Manual testing (rendered the README in GitHub preview to confirm headings, bullets, image, and report links display correctly)

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