Add a File tab for browsing files #22

Add a File tab for browsing a channel's files read-only: a file tree, and opening a file to view it. FileTree and FileView are unused.

Out of scope: editing, saving, creating and deleting files. They add complexity for little value, since agents and the shell already change files.

Dogfooding core/QOL: #5. Also tracked in #6 for old-Ace feature parity. Migrated from TODO.md — Port from old Ace.