docs: rewrite README OpenClaw install — one paste, real instructions (#818)

OpenClaw section promoted to peer of Claude Code install. Single copy-paste
prompt that installs gstack for Claude Code AND wires up AGENTS.md dispatch.
Usage table shows what happens when you talk naturally. Other agents collapsed
from repeated git clones into one auto-detect command + table. Voice input
moved after 10-15 parallel sprints section.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>