Turn runtime-core index.ts into a thin public API surface

[chubes4]

Goal

Make packages/runtime-core/src/index.ts read like the package's public API surface instead of a mixed implementation file.

Current Problem

Many core contracts and helpers have been extracted, but index.ts still contains substantial implementation and exported definitions. This makes the public API harder to reason about and encourages future changes to land in the entrypoint.

Target Shape

• index.ts primarily re-exports focused modules.
• Remaining implementation moves to named modules with clear ownership.
• Public exports remain stable unless an intentional API change is documented.

Candidate Slices

• Remaining action adapter contracts/helpers.
• Runtime state/context helpers.
• Workspace/runtime path helpers.
• Any remaining schema/assertion helpers that are not true package-level glue.

Acceptance Criteria

• index.ts is materially smaller and reads as an export/orchestration surface.
• Public exports and package distribution behavior are preserved.
• Build, package distribution, and relevant runtime-core smokes pass.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafting this tracker from the current architecture cleanup state.