Make joining a network idempotent and shut down cleanly on every path

Running `tsunagi up` twice with the same arguments failed with "network ...
is already active", and then dropped the iroh endpoint without closing it.

A configured network is activated automatically at startup, so the second
run found it already up. `join_network` is declarative — "be a member of
this network" — so joining one that is already active now succeeds and
changes nothing. `activate_network` stays strict for callers that
specifically want to know whether an inactive network was started.

The CLI now closes the agent on the error path too, and handles SIGTERM as
well as Ctrl-C, so a service manager stopping the agent gets the same clean
shutdown an interactive user does.

Also documents the two lookups people conflate: resolving one endpoint's
address is iroh's public pkarr/DNS service and works today, which is why
`--peer <endpoint-id>` needs no address; finding who is in a network is this
project's `NetworkDiscovery` and is still static bootstrap only. Notes in
the README and the threat model that `n0` and `direct` publish this
endpoint's addresses to a public third-party service.

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

docs/threat-model.md

@@ -41,6 +41,12 @@ Read this before relying on anything here. The protocol is in
 - **Weak secrets.** This targets high-entropy secrets. There is no PAKE, so a
   short human passphrase can be guessed offline by anyone who can reach the
   handshake. Use `NetworkSecret::generate()`.
+- **Public address publication.** With `TransportPolicy::N0Defaults` or
+  `DirectOnly`, iroh publishes a signed record of this endpoint's addresses,
+  keyed by its endpoint id, to Number 0's public pkarr/DNS service, and
+  resolves peers through it. The network secret is never published and
+  membership cannot be inferred from a single record, but the endpoint's
+  existence and its addresses become public. `LocalOnly` publishes nothing.
 - **Addresses and metadata are observable.** Anyone able to watch the network
   sees addresses, timing and volume. Discovery backends see the
   `discovery_key` and the addresses published under it, which is enough to map