Let a session's transport say what it negotiated

ISshConnection and ISftpSession both carry Cipher now — the server-to-client
algorithm off SSH.NET's own ConnectionInfo, captured once because a rekey is
not an event that library raises — and TerminalWorkspace.GetSessionFacts hands
that plus the host key's algorithm back per session, without ever handing over
the connection itself. Nothing reads either yet; the status bar that will is
the next commit.

src/DodoSSH.Client.Ssh/SftpSession.cs

@@ -265,6 +265,17 @@ public interface ISftpSession : IRemoteFileStore
 {
     /// <summary>The host key that was accepted for this session.</summary>
     HostKeyPresentation HostKey { get; }
+
+    /// <summary>
+    /// The negotiated server-to-client encryption algorithm, e.g. <c>aes256-gcm@openssh.com</c>.
+    /// </summary>
+    /// <remarks>
+    /// The same fact <see cref="ISshConnection.Cipher"/> is, read the same way — off SSH.NET's
+    /// <c>ConnectionInfo.CurrentServerEncryption</c> once the handshake this session's own connect performed
+    /// has finished — and for the same reason: <c>SftpClient</c> derives from <c>BaseClient</c> exactly as
+    /// <c>SshClient</c> does, and rekeys are no more visible here than they are there. See that member's remark.
+    /// </remarks>
+    string Cipher { get; }
 }
 
 /// <summary>