Let a device leave a network, and start over

Joining was one command and leaving was nothing at all: a network went into
`state.sqlite` on the first `up` and stayed there, so a mistyped secret
left a second network beside the working one with no way to remove it but
editing the database by hand.

`tsunagi network` lists what this device belongs to. `tsunagi network leave
<id>` publishes a signed release first — while the agent is running and its
sessions are up — and only then deactivates the network and removes it. The
order is the whole point: signed state has no expiry, so the tombstone is
the only thing that ever frees the address and the name for the others, and
after the network is gone there is nothing left here to sign one with.
Peers pass it on, so a member that was away hears it from them rather than
from an agent that has already left.

With no agent running nothing can sign or send, and the command says so
instead of quietly succeeding: `--offline` drops the network locally and
says plainly that the others keep the old claim. The outcome always
distinguishes "published to nobody" from "not published at all", because
they leave the network in different states.

A network is named by its id, and a unique prefix will do. The name is
refused on purpose: two networks can share one — that is exactly the
situation this command exists for — and picking between them for the user
is how the wrong one gets left.

The author's version counter deliberately survives. Rejoining the same
network with the same key must continue above the release, or every replica
that holds the release would treat the new claim as stale and the returning
member would be invisible for good. The protocol key does not survive:
rejoining is joining, not resuming, and coming back with a key the network
was told to let go claims an identity nobody holds any more. Plugins learn
about it through a new `on_network_forgotten`, which is about what outlives
a session rather than what a deactivation tears down.

A released member also drops out of the roster `status` prints. The
tombstone stays in the record set — a replica that never heard of it would
otherwise reinstate the old claim — but listing an author that gave
everything up as a member made leaving look like a peer that had broken.

`tsunagi wipe` is the other half: it empties both directories, so the
device identity, every network, every signed record and everything a
protocol kept beside them go at once and the next start is a stranger. It
refuses while an agent holds the directory, and refuses a directory with no
`state.sqlite` in it, so a mistyped `--state-dir` cannot take somebody's
documents with it. Without `--yes` it only prints what it would remove and
what membership would be lost. It is not a goodbye and says so: leaving the
networks first is what frees their addresses.

The local control protocol is 8 — the socket carries a `Leave` request now,
since only the running agent can publish the release.

Exercised end to end against real agents: leaving by prefix released the
address to a connected peer, leaving by name was refused, `--offline` was
refused until asked for explicitly, wipe was refused while the agent ran,
and the directory afterwards had no identity in it.

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

crates/tsunagi-wg-quic/tests/wireguard.rs

@@ -589,6 +589,34 @@ async fn an_interface_that_is_only_in_memory_is_not_reported_as_a_fault() {
     b.shutdown().await;
 }
 
+#[tokio::test]
+async fn leaving_a_network_takes_its_protocol_key_with_it() {
+    let discovery = SharedMemoryDiscovery::new();
+    let (name, secret) = network("wg-leave");
+
+    let agent = WgAgent::spawn(&discovery, "ta").await;
+    let network_id = agent.agent.join_network(&name, &secret).await.unwrap();
+    let before = wait_until("the plugin has a key for it", || async {
+        Some(agent.plugin.overview(network_id)?.public_key)
+    })
+    .await;
+
+    agent.agent.leave_network(network_id).await.unwrap();
+
+    // Rejoining is joining, not resuming: the key was this agent's identity
+    // in a network it left, and everyone there was told to let the address
+    // it held go. Coming back with the same key would claim an identity the
+    // network has already released.
+    agent.agent.join_network(&name, &secret).await.unwrap();
+    let after = wait_until("the plugin has a key again", || async {
+        Some(agent.plugin.overview(network_id)?.public_key)
+    })
+    .await;
+    assert_ne!(before, after, "a fresh key, not the released one");
+
+    agent.shutdown().await;
+}
+
 #[tokio::test]
 async fn an_address_is_kept_across_a_restart() {
     let discovery = SharedMemoryDiscovery::new();