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

internal/cli/ssh/install.go

@@ -7,9 +7,11 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"os/signal"
 	"path/filepath"
 	"slices"
 	"strings"
+	"syscall"
 
 	"github.com/spf13/cobra"
 )
@@ -55,6 +57,17 @@ func install() *cobra.Command {
 				return err
 			}
 
+			// From here on a signal cancels the context, which
+			// sftp runs under, instead of ending the tool, so sftp
+			// ends and the working directory is still removed.
+			ctx, stop := signal.NotifyContext(
+				cmd.Context(),
+				syscall.SIGINT, syscall.SIGTERM, syscall.SIGHUP,
+			)
+			defer stop()
+
+			cmd.SetContext(ctx)
+
 			return add(cmd, args[0], args[1:], line)
 		},
 	}