refactor: extract artifact manifest contracts

[chubes4]

Summary

• Extract artifact manifest contracts and manifest hashing helpers from packages/runtime-core/src/index.ts into packages/runtime-core/src/artifact-manifest.ts.
• Re-export the new module from runtime-core so the existing public API remains available from the package entrypoint.
• Keep artifact bundle verification and runtime behavior unchanged while shrinking the runtime-core index surface.

Tests

• npm run build
• npm run artifact-contract-smoke
• npm run artifact-bundle-verifier-smoke
• npm run package-distribution-smoke
• npm run runtime-episode-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Extracted the focused artifact manifest contract/helper slice, ran targeted verification, and drafted this PR description for Chris to review.