Refactor runtime episode contracts

[chubes4]

Summary

• Extract runtime episode schema IDs, JSON schema, canonical digest payloads, and trace validation into runtime-episode-contracts.ts.
• Keep runtime-episode.ts focused on runner lifecycle, trace/artifact persistence, JSONL shaping, and runtime execution details.
• Preserve the existing public runtime episode exports from runtime-episode.ts and @chubes4/wp-codebox-core.

Closes #379.

Tests

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

Caveats

• Ran npm install because the initial build could not find node_modules/typescript/bin/tsc; reverted the incidental package-lock.json version churn afterward.
• This intentionally leaves episode execution, artifact persistence, and snapshot bundle writing in runtime-episode.ts to avoid over-splitting behavior that still belongs to the runner.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafting and verifying the focused runtime episode contract extraction; Chris remains responsible for review and merge.