Remove tun-setup and the attach path it served

The managed interface supersedes both. They go together because apart
they are useless: attaching needs an interface somebody prepared, and
tun-setup existed only to say how to prepare one.

This also corrects what the last commit's README claimed. It said the
manual route was needed on macOS and Windows; it was not, and could not
be. The recipe printed Linux `ip` commands, and a persistent TUN that a
second process can attach to is a Linux concept — macOS creates a utun
by opening a control socket and there is nothing to hand over. So those
platforms were never served by this path, and their honest state is that
a real interface waits on a provisioner, with --no-tun meanwhile.

Gone with it: the interface-existence check, the /proc/net/if_inet6
address inspection and its DAD flag decoding, and the --interface flag,
which had one mode left.

Kept: the check that the allocated IPv4 address is really on a local
interface. The agent now assigns that address itself, so the check is no
longer telling a user what to run — it verifies the outcome instead of
trusting it, which is worth keeping precisely because the assumptions
around Linux address behaviour have been wrong here more than once. Its
message says which interface should have had the address rather than a
command to run.

Boxing Up(UpArgs) is fallout: TunSetupArgs had been masking how much
larger that variant is than its siblings.

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

tests/wireguard.rs

@@ -537,9 +537,11 @@ async fn an_allocated_address_missing_from_the_host_is_reported() {
         reason.contains(&allocated.to_string()),
         "unexpected: {reason}"
     );
+    // The agent assigns the address itself, so the report says which
+    // interface should have had it rather than a command to run.
     assert!(
-        reason.contains("ip address add"),
-        "must name the fix: {reason}"
+        reason.contains(&a.plugin.overview(network_id).unwrap().interface),
+        "must name the interface: {reason}"
     );
 
     a.shutdown().await;