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

lib/version/src/version.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <string>
+
 namespace roro {
 
 constexpr const char* kProductName = "roro9stack";
@@ -9,4 +11,10 @@ constexpr const char* kProductName = "roro9stack";
 // that one file, and everything else comes from the build cache (issue #74).
 const char* versionString();
 
+// The chip this firmware is built for: "xtensa-lx7 esp32s3". "host" in the native tests.
+const char* architectureString();
+
+// What `uname` and IRC's /uname report: "roro9stack v0.22.0 xtensa-lx7 esp32s3".
+std::string unameString();
+
 }  // namespace roro