Refactor runtime episode runner

[chubes4]

Summary

• Extract runtime episode schemas, digest/validation helpers, and RuntimeEpisodeRunner into packages/runtime-core/src/runtime-episode.ts.
• Keep packages/runtime-core/src/index.ts as the public export/orchestration surface and re-export the new focused runtime episode module.
• Preserve existing runtime episode behavior while using the current shared artifact/object helper utilities from main.

Tests

• npm run build
• npm run runtime-episode-smoke
• npm run runtime-snapshot-restore-smoke
• npm run runtime-action-adapter-smoke
• npm run artifact-bundle-verifier-smoke
• npm run package-distribution-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Runtime-core refactor extraction, conflict resolution during rebase, and local verification; Chris remains responsible for review and merge.