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/faq.md

@@ -67,7 +67,7 @@ Yes, WireGuard: one tunnel to one server, set up by copying the client's `.conf`
 
 ## Can it log in to my server?
 
-Yes, over SSH: the [SSH App](/guide/ssh/) is a terminal on another machine, with a password or with a key the device makes for itself. `vim`, `top` and `less` work. One session at a time, and not at the same time as IRC: there isn't the memory for both.
+Yes, over SSH: the [SSH App](/guide/ssh/) is a terminal on another machine, with a password or with a key the device makes for itself. `vim`, `top` and `less` work. One session at a time, and not at the same time as IRC: there isn't the memory for both. `scp` in the [Shell](/guide/shell/) copies a file between the card and a server.
 
 ## How do I copy files to and from my phone?