Refactor runtime core contracts out of index

[chubes4]

Summary

• Advances Turn runtime-core index.ts into a thin public API surface #375 by moving runtime lifecycle/session contracts from packages/runtime-core/src/index.ts into focused runtime-contracts.ts.
• Keeps the public @chubes4/wp-codebox-core barrel exports intact via export *, while internal core modules import the contracts from their owning module.
• Updates architecture docs so maintainers land new runtime contract work in the focused module instead of growing index.ts.

Tests

• npm run build
• npm run package-distribution-smoke
• npm run artifact-bundle-verifier-smoke
• npm run runtime-episode-smoke
• npm run runtime-action-adapter-smoke
• npm run runtime-reference-index-smoke
• npm run artifact-contract-smoke

Caveats

• index.ts is still not a pure barrel because artifact bundle verification helpers remain there; this PR intentionally extracts only the next coherent runtime contract slice.
• Initial parallel smoke execution produced transient Playground port collisions (EADDRINUSE); affected smokes were rerun sequentially and passed.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the focused TypeScript refactor, updated docs, ran verification, and prepared this PR for Chris to review.