[codex] Renovate SACP README entrypoint

[aDragon0707]

Summary

• Reframes SACP as the central protocol hub with quick start, receipt shape, ecosystem map, status, and roadmap.
• Changes are limited to README.md.
• No code, package config, license file, or GitHub metadata changes are included.

Validation

• Content gate passed; 4 README URLs reachable; diff check passed; secret-like scan found no matches.
• Last local commit changes README.md only.
• Renovation receipt is kept locally under work/github-renovation/receipts/.

Review note

This is a draft PR for review. Some README drafts intentionally compress long-form material into a cleaner entrypoint; selected original detail can be merged back or moved to docs before final merge.