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

site/content/howto/ssh-key.md

@@ -17,7 +17,7 @@ Typing a password on a small keyboard, every time, gets old. With a key, the ser
    chmod 600 ~/.ssh/authorized_keys
    ```
 
-4. On the Cardputer, connect: **SSH → New connection**, `user@host`. It logs in without asking for anything.
+4. On the Cardputer, connect: **SSH → New connection**, `user@host`. It logs in without asking for anything. The same key is what [`scp`](/howto/scp/) uses.
 
 ## If it still asks for a password