refactor(product-domains): move pure product parsing policies

[limityan]

Summary

• Move MiniApp command and host allowlist policy helpers into bitfun-product-domains while keeping host dispatch execution in core.
• Move Startchat and Git function-agent AI response value parsing into bitfun-product-domains while keeping AI calls, JSON extraction, and error mapping in core.
• Add contract tests plus boundary-check and architecture-plan anchors, including the latest MiniApp customization metadata / permission-diff boundary.

Scope and Safety

• This is a structure-only owner-boundary refactor; no product behavior, UI, command, or runtime execution semantics are intended to change.
• bitfun-core still owns PathManager, host dispatch execution, worker / storage IO, customization draft storage and apply flow, Git / AI service adapters, prompt templates, and tool runtime.
• The docs now explicitly mark runtime owner migration as deferred so this PR cannot be mistaken for a completed MiniApp/function-agent runtime migration.

Validation

• node scripts/check-core-boundaries.mjs
• rustfmt --edition 2021 --check on the touched Rust files
• cargo test -p bitfun-product-domains --features product-full -- --nocapture
• cargo test -p bitfun-product-domains --no-default-features
• cargo check -p bitfun-core --features product-full
• cargo test -p bitfun-core miniapp -- --nocapture
• cargo test -p bitfun-core function_agent -- --nocapture
• git diff --check