Creates a read-only channel used to attribute imported tickets to their
original source. Useful when bringing historical tickets into Mantle
for unified search and reporting.

Channels created here are attribution-only:

• Replies must happen in the original system; the send path refuses
  public replies on these channels. Internal notes, status changes,
  and assignments still work.
• The channel cannot be configured or edited from the helpdesk UI.

Repeated calls with the same type are idempotent — the existing
channel is returned rather than a duplicate created.

To set up a fully-functional email or chat channel, use the helpdesk
settings UI instead.