Chat mode gets its own composer

macOS only. Chat mode is for code, and its composer now says so: typing @ offers only the thread's folders and files, a teammate handle typed in a chat no longer mails that agent, and new Add files / Add folder buttons let you attach without dragging from Finder.

• Add files and folders from the composer. A + menu in the Chat composer opens a chooser for files or for one folder. A picked folder is granted whole, exactly as a dropped one is, and everything goes through the same staging as a drop.
• @ reaches files, not teammates. In Chat mode, @ lists the thread's folders and files only. Agents are reached from Agent mode, so a handle typed in a coding chat can no longer wake a teammate by accident.