Restrict teammates from sending via shared channels

Problem
Admins cannot prevent teammates from accidentally sending messages using shared channels instead of their individual channel, risking cross-customer communication errors.

Request
Provide admin controls to restrict outbound messaging to individual channels only, preventing shared channels from being used for sending. This could be an inbox-level setting or channel access control.

Context
Current workaround (setting a blank default channel) is fragile and can be overridden by teammates.