docs(architecture): clarify runtime decomposition layers

[limityan]

Summary

• Establish a runtime decomposition architecture baseline that covers current state, target layers, interface/implementation separation, Product Assembly registration, provider categories, and architecture risks.
• Add the Agent Runtime SDK / Runtime Services detailed design, including target crate boundaries, typed service injection, Remote port boundaries, Product Assembly registration flow, Product Capability / Harness extension points, hooks/events, robustness, and verification strategy.
• Split the migration plan into an active execution plan and a completed-work archive so the live plan focuses on current owner migration rhythm, remaining HR queues, target crate creation gates, and acceptance criteria.
• Clarify that the next phase starts with Product Assembly / Runtime Services foundation work before Service / Agent Remote runtime migration, with Remote product-surface verification requirements.

Related

• Follow-up tracking issue: 跟踪 core runtime 架构拆解闭环 #970

Validation

• git diff --check -- docs/architecture/core-decomposition.md docs/architecture/agent-runtime-services-design.md docs/plans/core-decomposition-plan.md docs/plans/core-decomposition-completed.md
• git diff --cached --check
• pnpm run check:repo-hygiene
• Verified all H2/H3 headings in the touched docs use numbered section prefixes.
• Verified the branch is rebased onto GCWing/main at 57bbc9ad and remains a single commit over main.

Notes

• This PR changes architecture and planning documents only; it does not change runtime behavior, product logic, feature defaults, build scripts, or release shape.
• GitHub currently reports no checks in statusCheckRollup for this docs-only PR branch.