Put every network's address on the interface, and split up from join

Two networks on one agent, and only one of them worked. The interface
plan is exhaustive by contract — it is what the interface should carry and
nothing else — but the request that built it held a single address, so the
provisioner was told about the first network and took the second one's
address off, or never put it on. On the host that is a network whose
address the operating system has never heard of: the tunnel is up, the
status says all is well, and nothing routes. The request now carries every
address, which is also what takes one off when a network is left or
stopped.

The other half is the command line. `up --network X --secret Y` and
`network join` were two ways to do the same thing, and the one on `up`
could only be undone by restarting — which is how a network somebody left
came back, and how an invite line told the other side to start their agent
with a network baked into it. So they are one thing now, split the way the
system is: **`up` runs the agent** — the device's one process, serving
whatever it has joined, answering `status`, taking instructions — and
**`join` decides what it belongs to**, at any time, while it runs. `join`
is at the top level because it is what gets typed; `network join` is the
same command for anyone who likes the long form.

Every line that told somebody to type the old form is gone with it: the
invite after making a network, the lock error from a second `up`, the
empty-network hint in `id`, the README walkthrough and the WireGuard
document. The invite now prints the `join` line for the other machine and,
separately, the `up --peer` line for an agent that is not running yet —
two commands, because they really are two, and no amount of wording makes
starting an agent the same thing as joining a network.

Joining says how the network stood before: new, already here, or stopped
and now running again. That last one matters — joining is an instruction
to run it, so it undoes a stop, and a pause that ends without a word is a
pause nobody can rely on.

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

docs/wireguard.md

@@ -206,8 +206,9 @@ edit. Reconciliation is purely "do the running tunnels match what is known".
 ## Using it
 
 ```bash
-# On both machines
-tsunagi up --network lab --secret "$SECRET" --wireguard
+# On both machines: the agent, then the network
+tsunagi up
+tsunagi join --network lab --secret "$SECRET"
 ```
 
 See the two-machine walkthrough in [../README.md](../README.md#trying-it-on-two-machines).