Extract observation artifact helpers

[chubes4]

Summary

• Extract WordPress state observation export/artifact writing into observation-artifacts.ts.
• Extract HTTP response body artifact capture into the same observation-focused helper module.
• Keep index.ts responsible for observation routing while reducing the runtime class by about 330 lines.

Testing

• npm run build
• npm run runtime-episode-smoke
• npm run artifact-contract-smoke
• npm run package-distribution-smoke
• npm run command-registry-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the observation helper extraction, ran verification, and prepared the PR summary for Chris to review.