ax(node): replace prose comment with usage example on DeriveSharedSecret

AX Principle 2 — comments show HOW with real values, not WHAT the
signature already says. The old comment restated the method name;
the new comment shows the exact call pattern used in transport.go.

Co-Authored-By: Charon <charon@lethean.io>

pkg/node/identity.go

@@ -166,8 +166,8 @@ func (n *NodeManager) GenerateIdentity(name string, role NodeRole) error {
 	return nil
 }
 
-// DeriveSharedSecret derives a shared secret with a peer using X25519 ECDH.
-// The result is hashed with SHA-256 for use as a symmetric key.
+// secret, err := nm.DeriveSharedSecret(peer.PublicKey)
+// smsg.Encrypt(msg, base64.StdEncoding.EncodeToString(secret))
 func (n *NodeManager) DeriveSharedSecret(peerPubKeyBase64 string) ([]byte, error) {
 	n.mu.RLock()
 	defer n.mu.RUnlock()