Refactor content native message boundary

[Kingson4Wu]

Summary

• extract content-script native runtime message helpers into contentNativeMessages
• keep content.ts focused on YouTube page bootstrapping and UI/session wiring
• add an architecture guard for the content native messaging boundary

Verification

• pnpm run typecheck
• pnpm test

Notes

• Loop architecture assessment still reports score 92/100 because .codegraph is absent from required guard files. Project instructions say CodeGraph indexing is user-owned, so this PR does not create .codegraph.