docs(readme): quarantine legacy openclaw setup examples

[ToadAid]

Summary

• add a small Mirror-native entrypoint block near the top of the README
• keep the existing mirror-runtime repo-boundary note in place
• relabel the existing OpenClaw install, quick-start, and from-source examples as upstream or compatibility context so they are no longer the first actionable examples at the repo front door

Verification

• pnpm format:check

[ToadAid]

Closing this in favor of #236.

This branch hit a README rebase conflict after main moved. I rebased the patch cleanly, preserved the
same docs-only change, and pushed the resolved state to a replacement branch/PR because force-pushing
this branch is blocked by repo rules.

Replacement PR: #236