Cowork: folder mount unavailable until after first message in a new session

Description

When starting a new Cowork chat, the "mount folder" option isn't available on the empty/pre-session screen. I have to type and send a message first before the mount-folder control becomes active.

Expected behavior

Ability to mount a working folder before sending the first message, so the session starts with the right directory context from turn one instead of needing a throwaway first message.

Current workaround

Send any placeholder message, then mount the folder once the session UI unlocks it.

Guess at root cause

The folder-mount picker likely binds to a live session/container id, and that container isn't provisioned until the first message triggers session start — so there's nothing for the picker to bind to pre-session.