refactor(product-domains): move function agent response policy

[limityan]

Summary

• Move function-agent prompt templates into bitfun-product-domains while preserving the existing prompt text byte-for-byte.
• Move Git commit-message and Startchat work-state AI response JSON extraction, JSON repair, JSON-to-domain parsing, and domain error mapping policy into bitfun-product-domains::function_agents.
• Keep concrete Git/AI service adapters, provider acquisition, AI client calls, AI transport error handling, and runtime orchestration in bitfun-core.
• Update decomposition docs, product-domains/AGENTS.md, and the boundary check script so H2 no longer describes prompt/response policy as core-owned while still keeping MiniApp IO/worker/host/builtin runtime and Git/AI runtime explicitly core-owned.

Behavior Boundary

This PR is intended as a structural owner migration only. It does not move MiniApp filesystem IO, worker process execution, host dispatch, built-in asset seeding or marker IO, Git service execution, AI client calls, tool runtime, or product-surface behavior.

Verification

• node scripts/check-core-boundaries.mjs
• git diff --check gcwing/main..HEAD
• cargo test -p bitfun-product-domains --features function-agents function_agents:: --lib
• cargo test -p bitfun-core parse_commit_response_preserves_product_domain_response_policy --lib
• cargo test -p bitfun-core parse_complete_analysis_preserves_product_domain_response_policy --lib
• cargo check --workspace
• cargo test --workspace