One tool, three actions backed by FabricClient channelMembers (GET
/channels/:id/members -> [{userId,bypass}]), joinChannel, and new
leaveChannel (POST /channels/:id/leave).
Verified: client-level smoke against the running guild — members
initial=[tester], after join echo2 present, after leave echo2 gone.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>