Stored as slack_channel_enrollments.mode. A channel with no enrollment
row is implicitly MANUAL; storing an explicit MANUAL row lets the
channel carry per-channel state (like custom_instruction) without
enabling auto-triggers. Removing a channel from Channel Management entirely
is a separate act (archiving the enrollment), not a mode.
READ_ONLY is the one mode that suppresses outbound behavior rather
than selecting a flavor of it: Traversal keeps ingesting and storing every
message, and answers every interactive surface with a fixed notice instead
of doing work. It is only offered while its organization's gate is on --
see engagement_policy. Existing READ_ONLY channels keep the mode
when that gate is turned back off, so nothing starts talking again behind
a customer's back.