Add loop-swarm subsection to QUICKSTART #424

Goal

Add a loop-swarm subsection to QUICKSTART so multi-agent consensus sandboxing is discoverable next to loop-sandbox / L2 safety tools.

Files

• docs/QUICKSTART.md — short subsection after loop-sandbox (or under L2/L3 safety)
• Cross-link tools/loop-swarm/README.md and docs/safety.md

Acceptance criteria

• Explains sequential multi-agent runs + majority byte-identical consensus in 1–2 paragraphs
• Shows a copy-paste example: npx @cobusgreyling/loop-swarm run --count 3 -- <agent-cmd>
• Notes limitations (serialized runs, SIGINT, not an OS sandbox) with a link to safety docs
• Does not rewrite the unified @cobusgreyling/loop front door

Estimated time: ~25–30 minutes

Comment "I'll take this" to get assigned.