A system channel is a channel used only for access control, for example
to hold config that only an agent and its owner can read. This commit adds
the type; later commits leave it out of queries unless a filter asks.
Migration 0062 adds 'system' to the channel_type enum, and buzz-core adds
ChannelType::System. kind:9007 with ["channel_type", "system"] creates
one. It is private unless the creator sends ["visibility", "open"]. Its
kind:39000 carries NIP-29 "hidden" while it is private, from the relay and
from buzz-admin reconcile. Usage metrics count system channels.
Signed-off-by: Logan Johnson <loganj@squareup.com>
Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
