Log inbound handshake rejections at debug, not warn

Creating and leaving networks produced a stream of WARN lines like "Handshake
rejected: peer requested an unknown or inactive network network=None". This is
not a leave bug: leaving deactivates the network and publishes a release, but
remote peers that still consider this device a member keep dialing, and each
dial for a network no longer served is correctly rejected by the responder
(lookup returns None, so the network stays unidentified — network=None by
design, since a pre-handshake claim is unverified).

Such a rejection is routine and remotely triggerable — a left network a peer
has not caught up on, a stranger, a port scan, an old client — so logging it at
WARN let any peer spam the log. It is now debug; enable debug to see rejections
when diagnosing. The rejection behaviour itself is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/tsunagi-cli/src/main.rs

@@ -4435,7 +4435,12 @@ fn log_event(event: &Event) {
             let peer_str = peer
                 .map(|p| p.fmt_short().to_string())
                 .unwrap_or_else(|| "unknown".into());
-            tracing::warn!(
+            // Routine and remotely triggerable: a peer dialing a network this
+            // agent does not serve — one it never had, or has left and the peer
+            // has not learned yet — a stranger, or an old client. Rejecting is
+            // correct and contained (one session), so this is debug rather than
+            // a warning any peer could spam into the log.
+            tracing::debug!(
                 network = ?network.map(|n| n.fmt_short().to_string()),
                 peer = %peer_str,
                 "Handshake rejected: {reason}"