Improve README demo proof

[prashar32]

Summary

• Add a See it work first section near the top of the README with the no-key Docker demo and expected HTTP 402 output.
• Add a crash-resume terminal recording and rendered GIF for examples/kill-9-resume.
• Add a Pick a demo table so visitors can quickly choose the right example.
• Remove the older buried demo paragraphs after the SDK quickstart to reduce README friction.

Why

The promotion flow needs the repository landing page to convert faster. New visitors should be able to see the runaway-agent proof, the crash-resume proof, and the right demo path without reading the whole README first.

Validation

• env GOCACHE=/private/tmp/riskkernel-gocache go test ./internal/governor ./internal/runs ./internal/storage
• git diff --check
• Rendered and visually checked examples/kill-9-resume/resume.gif from the existing demo script.