refactor(runtime): centralize core runtime owners

[limityan]

Summary

This PR consolidates the current HR2 + HR3 runtime-owner closure work into a single commit after rebasing onto the latest GCWing/BitFun:main.

What changed:

• Add CoreProductDomainRuntime as the core-owned binding point for product-domain runtime adapters.
• Add CoreServiceAgentRuntime as the core-owned binding point for service/agent runtime adapters.
• Route existing MiniApp/function-agent, remote dialog, remote image-context, and agent submission bindings through those owner entry points.
• Update boundary checks and module docs so deferred runtime owners remain explicit and reviewable.

What this intentionally does not change:

• No Cargo feature/default behavior changes.
• No remote-connect, remote-SSH, scheduler, registry, MiniApp worker, Git/AI, or terminal runtime ownership migration.
• No product-surface, wire-shape, CLI/Desktop/Remote/ACP behavior changes.

Risk Review

The main risk would be accidentally treating core-owned runtime paths as migrated owner-crate behavior. This PR keeps those paths in core and only centralizes the adapter entry points. The added boundary rules and focused tests guard the current ownership line so future runtime migration needs a separate port/provider design and equivalence tests.

Validation

• rustfmt --edition 2021 --check --config skip_children=true ...
• node scripts/check-core-boundaries.mjs
• git diff --check gcwing/main..HEAD
• cargo test -p bitfun-core core_product_domain_runtime_owner -- --nocapture
• cargo test -p bitfun-core core_service_agent_runtime_owner -- --nocapture
• cargo test -p bitfun-core remote_ -- --nocapture
• cargo test -p bitfun-product-domains --features product-full
• cargo test -p bitfun-services-integrations --features product-full
• cargo check -p bitfun-core --features product-full