refactor(runtime-ports): extract dialog and subagent contracts

[limityan]

Summary

• Move portable dialog submission contracts into bitfun-runtime-ports: trigger source, queue priority, submission policy, and submit outcome.
• Move portable subagent contracts into bitfun-runtime-ports: delegation policy and subagent context mode.
• Keep core as the runtime owner through compatibility re-exports, while expanding boundary checks and docs for latest-main Task, tool, and MCP migration guardrails.

Behavior Boundaries

• No scheduler, coordinator, session branch, prompt cache, background delivery, or tool execution ownership is moved in this PR.
• Core import compatibility is preserved through re-exports.
• Queue priority, skip-confirmation defaults, fork-context validation, background acknowledgement structure, prompt-cache clone, truncation recovery, and denial-message behavior are covered by tests or boundary checks.

Validation

• node scripts\check-core-boundaries.mjs
• git diff --check gcwing/main...HEAD
• cargo check --workspace
• cargo test --workspace

Follow-up Scope

• HR-C: continue service/agent runtime migration only after adding equivalence coverage for scheduler/session restore, request-context assembly, prompt compression/cache, goal verification events, and background delivery timing.
• HR-A: if tool runtime migration resumes, cover Write sanitizer, AskUserQuestion/TodoWrite truncation recovery, per-tool denial messages, tool-result flush, runtime artifact persistence, and concrete tool IO boundaries.
• H5: evaluate feature/build benefit separately with CLI package workflow and mobile-web session operations in the product matrix; do not mix feature-set changes into runtime-owner migration.