Until now collectives only existed at registration; an account with a
personal workspace had no path to start one. The creator becomes Owner
and gets a session scoped to the new workspace.
The live tuwunel does not offer m.login.jwt (the jwt.* flat config keys were
inert), so the JWT broker flow could never work. Pivot: appservice.hub owns
the exclusive hub_* namespace on the homeserver; POST /matrix/session now
registers @hub_<username> with the as_token and mints a single-use
m.login.token via login/get_token (?user_id= impersonation). Optional
MATRIX_SSO_HS_TOKEN authenticates inbound appservice transactions, acked by
the new MatrixAppserviceController sink. Response shape unchanged.
