refactor(product-domains): move runtime owner helpers

[limityan]

Summary

• Move pure MiniApp built-in seed artifact/action resolution and install-marker wire helpers into bitfun-product-domains, while keeping asset includes, filesystem IO, marker writes, customization metadata IO, timestamping, and recompile orchestration in core.
• Move Git commit-message and Startchat complete-analysis JSON-string parsers into bitfun-product-domains, while keeping prompt templates, AI calls, JSON extraction, logging, and error mapping in core.
• Update module docs and scripts/check-core-boundaries.mjs so future work does not mistake these owner helpers for runtime/IO migration completion.

Safety / behavior boundary

• No MiniApp worker execution, host dispatch, storage IO, asset seeding IO, or PathManager ownership moved.
• No Git/AI service runtime, prompt template, JSON extraction, or concrete error mapping moved.
• Added focused contract/regression coverage for MiniApp seed plan/marker wire shape and function-agent parser delegation.

Validation

• git fetch --no-tags git@github.com:GCWing/BitFun.git main:refs/remotes/gcwing/main
• git rebase --autostash gcwing/main
• node scripts/check-core-boundaries.mjs
• git diff --check
• cargo test -p bitfun-product-domains --features product-full
• cargo check -p bitfun-core --features product-full
• cargo test -p bitfun-core builtin_reseed --lib
• cargo test -p bitfun-core parse_commit_response_preserves_core_json_extraction_and_error_mapping --lib
• cargo test -p bitfun-core parse_complete_analysis_preserves_core_json_extraction_and_error_mapping --lib