Decide how to route to a cluster per connection, not at cluster add time

miren cluster add decides once, at add time, whether a cluster is reached directly or through Miren Cloud, and records that in the client config.

That is wrong for a client that moves. A laptop that adds a cluster from the office records a direct route, then fails on the same cluster from a coffee shop, and vice versa: a cluster added while remote stays routed through cloud even when the user is back on a network that can dial it directly, which is slower for no reason.

The decision should be made when the client connects, not when the cluster is added. Roughly: try direct, fall back to cloud, and cache the answer briefly rather than permanently.

Raised in review of RFD-101 (Cluster RPC via Cloud), which records it as a known limitation of the current design.