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/_index.md

@@ -1,6 +1,6 @@
 +++
 title = "How-tos"
-description = "Short recipes for things you will want to do: move files with your phone, set up the VPN, log in to a server with the device's SSH key, take a screenshot, find out why the network doesn't work, record a track, capture radio packets, and what to try when something does not work."
+description = "Short recipes for things you will want to do: move files with your phone, set up the VPN, log in to a server with the device's SSH key, copy a file to or from a server, take a screenshot, find out why the network doesn't work, record a track, capture radio packets, and what to try when something does not work."
 template = "guide-index.html"
 page_template = "guide-page.html"
 sort_by = "weight"