Re-shoot the guide's screenshots and add one for migrate

shots.py gains a pseudo-terminal runner so it can answer migrate's
overwrite question, and the guide shows the resulting screenshot.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/guide/vault-guide.tex

@@ -572,6 +572,8 @@ before touching anything. Answer anything but \texttt{y} and nothing changes.
 With no terminal to ask on, the answer is no. For scripts, \texttt{--yes}
 answers yes up front.
 
+\shot{migrate}{Moving a folder in. One file was already there, so vault asked first.}
+
 \subsection{What gets skipped}
 When adding a folder, anything that isn't a plain file, such as symbolic
 links, sockets or devices, is skipped with a note. So are the vault and key