Add uname, /uname and scp (key or masked password) in the Shell

- uname in the console and /uname in IRC: the firmware, its version and the chip
- scp: one file between the card and a trusted server, over SSH, with the
  device's key or a password asked (masked) in the Shell
- shared libssh helpers in src/platform/libssh_util
- README, user guide, command reference and a how-to updated

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0162FokPdvY2KsS4NBfwyWPk

src/platform/libssh_util.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <string>
+
+#include <libssh/libssh.h>
+
+// What the SSH Service and the Scp Service both do with libssh.
+namespace roro::sshutil {
+
+// libssh_begin(), once.
+void startLibrary();
+// "SHA256:..." of the key the server showed, or "".
+std::string fingerprintOf(ssh_session s);
+
+}  // namespace roro::sshutil