docs: refine agent-native adoption candidate

[chasechou007]

Summary

• add alpha.8 standalone and registry evidence to Candidate 0012
• record OpenSpec as external interaction-pattern evidence
• remove Homebrew as a promotion prerequisite while retaining a real-project adoption gate

Validation

• npm run opendomain -- validate
• openspec validate --all --json

Candidate 0012 remains proposed; this PR does not promote accepted domain knowledge.