fix(runner): avoid duplicate system prompt on new OpenClaw

[YOMXXX]

Summary

• avoid passing extraSystemPrompt on OpenClaw versions that support agents.defaults.systemPromptOverride
• preserve the legacy extraSystemPrompt fallback for unknown or older OpenClaw versions
• add unit coverage for the version gate
• keep the npm tarball under Size Guard by publishing built runtime artifacts only

This is intended as a compatibility-preserving alternative to #84.

Verification

• npm test
• npm run build
• npm pack --dry-run (package size: 340.4 kB, unpacked: 1.3 MB)