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>

src/discovery.rs

@@ -8,14 +8,17 @@
 //! A discovery backend must not carry control messages between agents, must not
 //! confirm authentication and must not mutate agent state directly.
 //!
-//! Two concerns are kept apart:
+//! Two concerns are kept apart, and only the first one is this module's:
 //!
 //! * *Finding members of a network* — [`NetworkDiscovery::resolve`], keyed by
-//!   the secret-derived [`DiscoveryKey`].
-//! * *Resolving the address of one iroh endpoint* — an
-//!   [`iroh::EndpointAddr`] either already carries addresses, or iroh's own
-//!   address lookup service must be enabled. Dialling a bare [`EndpointId`]
-//!   with neither is expected to fail.
+//!   the secret-derived [`DiscoveryKey`]. That is what lives here, and today
+//!   it is [`StaticBootstrap`] plus a test backend; a DHT backend is future
+//!   work.
+//! * *Resolving the address of one iroh endpoint* — **iroh's job, not ours**.
+//!   With [`crate::config::TransportPolicy::N0Defaults`] or `DirectOnly`, iroh
+//!   publishes and resolves endpoint addresses through Number 0's public
+//!   service, so dialling a bare [`EndpointId`] works. With `LocalOnly` there
+//!   is no lookup, and a candidate must carry addresses of its own.
 //!
 //! No empty result ever proves a network is empty. It only means "nobody found
 //! yet".