docs: rewrite README — clear user journey, setup-first flow

Reorganize from feature-dump to user journey:
- Install and Quick start at the top
- Each agent gets its own focused section
- OpenClaw section accurately describes bridge + shim + patches
- OWASP coverage condensed to a table at the bottom
- CLI reference trimmed to what people actually use
- 50% shorter overall

Author: clap [bot]