Polish the README and simplify repository docs

[Praket7]

Summary

• Rewrite the README with plain language and direct versioned downloads for every supported platform
• Add two generated README illustrations and keep the accurate sample report
• Remove duplicate build notes and the unused mark asset
• Correct ignored binary names

Verification

• Confirmed every release archive matches the published SHA 256 manifest
• Ran the published macOS arm64 binary through the mock provider and OpenCode fixture
• Checked the diff for whitespace errors
• Go source checks are left to GitHub Actions because Go is not installed in this checkout