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/irc/src/irc_session.cpp

@@ -360,10 +360,12 @@ void IrcSession::command(int b, const std::string& text, int64_t utc) {
     } else if (verb == "quit") {
         quit_ = true;
         send(IrcMessage::serialize("QUIT", {rest.empty() ? "roro9stack" : rest}));
+    } else if (verb == "uname") {  // shown here, not said to anyone
+        info(b, unameString(), utc);
     } else if (verb == "raw" || verb == "quote") {
         if (!rest.empty()) send(rest);
     } else {
-        info(b, "Unknown command /" + verb + " (try /join or /j, /part /msg /me /nick /topic /names /quit /raw)", utc);
+        info(b, "Unknown command /" + verb + " (try /join or /j, /part /msg /me /nick /topic /names /uname /quit /raw)", utc);
     }
 }