[Stage 3 / P2] refactor: split slash.js into interaction/ + feedback/ subsystems

[modacker]

[Stage 3 / P2] refactor: split slash.js into interaction/ + feedback/ subsystems

Mirror of MiniMax-AI/MiniMax-Code-Plugins#27

范围

• split slash.js into interaction/ + feedback/ subsystems
• 借鉴 dsh borrow [Stage 2 / P1] UX: slash command autocomplete + settings card grouping #3
• 估算 ~600 行

cc @Wzdhehe