fix(assist): keep visual dispatch payload within GitHub limit

[Takhoffman]

Summary

• remove the unused assist dispatch item_kind payload field so repository_dispatch stays within GitHub's 10-property limit
• centralize assist dispatch payload construction for text and visual assist
• add regression coverage for the visual payload property count

Validation

• pnpm run build:repair
• node --test test/repair/comment-router-core.test.ts
• pnpm run format:check
• pnpm run check