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.

tests/DodoSSH.Client.Transfer.Tests/FakeSftpSession.cs

@@ -23,6 +23,9 @@ internal sealed class FakeSftpSession : ISftpSession
     /// <inheritdoc />
     public HostKeyPresentation HostKey { get; } = new("host.internal", 22, "ssh-ed25519", "SHA256:fake");
 
+    /// <inheritdoc />
+    public string Cipher { get; } = "aes256-gcm@openssh.com";
+
     /// <inheritdoc />
     public string HomeDirectory => "/home/dodo";