[ WilliamPenrose/snap-protocol ] Rewrite README for clarity and viral spread

- New tagline: "AI agents authenticate themselves with a keypair instead of API keys"
- Add "How it works" JSON example, Quick Start (sign + verify), Bitcoin FAQ
- Fix sig format to valid Schnorr hex, fix "zero security impact" overclaim
- Add MCP/A2A positioning in "Relationship to other protocols" section
- Condense Skills install, move A2A comparison table to design-decisions.md
- README reduced from 237 to 206 lines

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
https://github.com/WilliamPenrose/snap-protocol/commit/0a34fed05acb948be5d5348cd6b1969b3a21ba35