Signals end every command, not only ssh to and ssh install (closes #48)

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

README.md

@@ -274,6 +274,15 @@ girl mad pet galaxy egg matter matrix prison refuse sense ordinary nose
 Errors go to standard error and the exit status is 1, except for `ssh to`, which
 passes through `ssh`'s own exit status.
 
+SIGINT, SIGTERM and SIGHUP end any command at once, at the mnemonic prompt too,
+with the status a shell gives a program killed by that signal (130 for SIGINT).
+An interrupted `age encrypt -o` or `age decrypt -o` leaves the named file as it
+was; the unfinished file it was writing stays beside it, named
+`<file>.<digits>`. While `ssh to` or `ssh install` has `ssh` or `sftp` running,
+the signal ends that program instead, the tool removes its agent socket or
+working files, and it exits with status 1, or for `ssh to` with `ssh`'s own
+status if `ssh` reported one.
+
 ## Entrypoints
 
 The repo adheres to the