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/SshNetSftpSession.cs

@@ -20,6 +20,14 @@ internal sealed class SshNetSftpSession(SftpClient client, HostKeyPresentation h
     /// <inheritdoc />
     public HostKeyPresentation HostKey { get; } = hostKey;
 
+    /// <inheritdoc />
+    /// <remarks>
+    /// Read at construction, the same way and for the same reason as <c>SshNetConnection.Cipher</c>: this type
+    /// is only ever built after <c>SshNetConnectionFactory.OpenSftpAsync</c> has awaited <c>client.ConnectAsync</c>,
+    /// so <c>ConnectionInfo</c> is already populated by the time there is a session to read it from.
+    /// </remarks>
+    public string Cipher { get; } = client.ConnectionInfo.CurrentServerEncryption;
+
     /// <inheritdoc />
     public string HomeDirectory { get; } = homeDirectory;