Extract runtime action adapter from core index

[chubes4]

Summary

• Advances Turn runtime-core index.ts into a thin public API surface #375 by moving the runtime action adapter contracts and helpers out of packages/runtime-core/src/index.ts into runtime-action-adapter.ts.
• Keeps the existing public entrypoint exports intact via export * while leaving index.ts closer to a thin public API surface.
• Reverted incidental package-lock.json version churn from npm install.

Tests

• npm run build
• npm run package-distribution-smoke
• npm run runtime-action-adapter-smoke

Caveats

• This is one focused slice toward Turn runtime-core index.ts into a thin public API surface #375, not the final thin-index cleanup.
• The new adapter module still type-imports core runtime contracts from the public index until those contracts are extracted in a later slice.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Extracting the runtime action adapter slice, running verification, and drafting this PR description. Chris remains responsible for review and merge.