Refactor Rust modules: keep mod.rs files import/export-only #559

Summary

Refactor Rust module structure so all mod.rs files remain import/export focused and operational logic lives in sibling module files.

Problem

Some mod.rs files contain implementation logic, making modules harder to navigate and maintain.

Scope

• Audit src/openhuman/**/mod.rs files.
• Move operational code into focused sibling files (e.g., ops.rs, store.rs, types.rs, rpc.rs).
• Keep mod.rs limited to mod declarations and re-exports.
• Preserve existing behavior and public API surface.

Acceptance Criteria

• mod.rs files are export-focused with minimal/no operational logic.
• Functionality and tests remain unchanged after refactor.
• Module boundaries are clearer and easier to maintain.