SIGINT, SIGTERM and SIGHUP were caught for the whole run, but only the
ssh and sftp children acted on them: the mnemonic prompt waited for
Enter, and an interrupted `age encrypt -o` went on to put the
encryption of the cut-off input in place. Now only `ssh to` and
`ssh install` catch them, from once the mnemonic is read until their
cleanup has run; everywhere else they end the tool at once. Tests cover
an interrupted `age encrypt -o` and the install working directory on a
signal.
Model: opus-5-5
