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 `ssh to` and
`ssh install` catch them from once the mnemonic is read until their
cleanup has run, and `age encrypt -o` and `age decrypt -o` catch them
while they write, to remove the unfinished file and exit with status 1;
everywhere else they end the tool at once. Tests cover an interrupted
`age encrypt -o` and `age decrypt -o` and the install working directory
on a signal.

Model: opus-5-5

internal/cli/age/age.go

@@ -6,7 +6,9 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"os/signal"
 	"path/filepath"
+	"syscall"
 
 	"github.com/spf13/cobra"
 	"sneak.berlin/go/keyfunc/internal/agekey"
@@ -185,16 +187,38 @@ func output(cmd *cobra.Command) (io.Writer, func(error) error, error) {
 		}, nil
 	}
 
+	// From before the new file is made until it is renamed or removed,
+	// a signal removes it and ends the tool with status 1, even while
+	// the work is blocked reading its input, so an interrupted run
+	// leaves no file.
+	signals := make(chan os.Signal, 1)
+	signal.Notify(signals, syscall.SIGINT, syscall.SIGTERM, syscall.SIGHUP)
+
 	// The file is made in the same directory so that putting it in
 	// place is a rename and never a copy, and it is readable only by
 	// its owner, which is the mode it keeps once renamed.
 	file, err := os.CreateTemp(filepath.Dir(name), filepath.Base(name)+".")
 	if err != nil {
+		signal.Stop(signals)
+
 		return nil, nil, fmt.Errorf("creating a file beside %s: %w", name, err)
 	}
 
+	go func() {
+		if _, received := <-signals; received {
+			_ = os.Remove(file.Name())
+
+			os.Exit(1)
+		}
+	}()
+
 	return file, func(failed error) error {
-		return finish(file, name, failed)
+		finished := finish(file, name, failed)
+
+		signal.Stop(signals)
+		close(signals)
+
+		return finished
 	}, nil
 }