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.cpp

@@ -0,0 +1,29 @@
+#include "platform/libssh_util.h"
+
+#include <libssh_esp32.h>
+
+namespace roro::sshutil {
+
+std::string fingerprintOf(ssh_session s) {
+    ssh_key key = nullptr;
+    unsigned char* hash = nullptr;
+    size_t len = 0;
+    std::string out;
+    if (ssh_get_server_publickey(s, &key) == SSH_OK && ssh_get_publickey_hash(key, SSH_PUBLICKEY_HASH_SHA256, &hash, &len) == SSH_OK) {
+        if (char* text = ssh_get_fingerprint_hash(SSH_PUBLICKEY_HASH_SHA256, hash, len)) {
+            out = text;  // "SHA256:..."
+            ssh_string_free_char(text);
+        }
+        ssh_clean_pubkey_hash(&hash);
+    }
+    if (key) ssh_key_free(key);
+    return out;
+}
+
+void startLibrary() {
+    static bool started = false;
+    if (!started) libssh_begin();
+    started = true;
+}
+
+}  // namespace roro::sshutil

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