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>

README.md

@@ -72,6 +72,7 @@ On the first machine:
 cargo build --release
 ./target/release/tsunagi id secret generate   # prints tsn1...; share it privately
 ./target/release/tsunagi status          # this device, the agent, and this host
+./target/release/tsunagi network         # the networks this device belongs to
 
 ./target/release/tsunagi up --network lab --secret "$SECRET"
 ```
@@ -495,6 +496,36 @@ directories by default; `--state-dir` and `--cache-dir` override them.
 One state directory belongs to one live agent, enforced with a real OS file
 lock rather than an existence check.
 
+### Leaving, and starting over
+
+Membership outlives a session, so it also has to be possible to end it.
+
+```bash
+tsunagi network                     # what this device belongs to
+tsunagi network leave <network-id>  # give up the address and the name, then forget it
+tsunagi wipe --yes                  # remove everything and be a stranger again
+```
+
+`leave` publishes a signed `Release` **first**, while the agent is running and
+its sessions are up, so the address and the name are freed for the others
+instead of staying reserved to a member that has gone. They pass the tombstone
+on, so a member that was away hears it from them. With no agent running,
+nothing can sign or send it: the command says so and refuses, and `--offline`
+drops the network locally while leaving the others holding the old claim. The
+protocol key for that network goes too — rejoining is joining, not resuming.
+
+A network is named by its id, never by its name: two networks can share a
+name, and choosing between them for the user is how the wrong one gets left. A
+unique prefix is enough.
+
+`wipe` removes both directories' contents: the device identity, every network,
+every signed record and everything a protocol kept beside them. It refuses
+while an agent is running, 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 says what it would remove. It is not a goodbye: nobody
+is told, because after it there is no key left to sign anything with. Leave the
+networks first if the addresses should be freed.
+
 ## Documentation
 
 - [docs/architecture.md](docs/architecture.md) — module boundaries and runtime.