[codex] Add open-source README overview

[Pigbibi]

Summary

• Add a bilingual investment-risk notice near the top of the README.
• Add an open-source oriented overview explaining what the repository does, current status, quick start, safe deploy/operation path, and evidence/performance boundary.
• Keep the existing detailed runbooks and historical notes below the new entry section.

Validation

• README coverage script checked Chinese and English risk notices plus overview, quick start, deploy/operate, and evidence sections.
• git diff --check

Notes

This PR is documentation-only and does not change runtime behavior, workflows, credentials, or strategy defaults.

[chatgpt-codex-connector]

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