Start the agent without deciding anything yet

`up` demanded a network, so there was no way to run the agent in one
terminal and decide what it belongs to in another — which is the shape of
the thing now that networks are joined, stopped and left while it runs.

`--network` is optional. Without it the agent starts with whatever it is
already configured for and waits; the banner counts the networks instead of
naming one, and points at `tsunagi network join`. A secret with no network
is refused rather than ignored, because it says nothing on its own.

The periodic summary, which had one network by construction, now falls back
to the first running one — `tsunagi status` is the whole picture and this
line was never more than a glance.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -370,6 +370,11 @@ A secret is printed by `network secret` and nowhere else — not by `id`, not
 by `status`, not in a log, a `Debug` rendering or anything sent to a peer.
 Asking for it is deliberate, because these reports get pasted into chats.
 
+**`up` takes no network at all** if you would rather decide later: it
+brings up the agent and whatever it is already configured for, and waits.
+That is the shape of a daemon in one terminal and `tsunagi network join`
+in another.
+
 `network join` is also the answer to a question `up` cannot: a state
 directory belongs to one live agent, so a second `tsunagi up` cannot add a
 network to the one already running. This adds it over the control socket and