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` 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 everywhere
else they end the tool at once, except while `age encrypt -o` or
`age decrypt -o` writes. There the work runs in the background, and a
signal that comes before it ends, or within a tenth of a second after,
removes the unfinished file and ends the tool with status 1, since
Ctrl-C on a pipeline can end the input just before the signal arrives.

Model: opus-5-5

internal/cli/cli.go

@@ -2,13 +2,10 @@
 package cli
 
 import (
-	"context"
 	"errors"
 	"fmt"
 	"os"
-	"os/signal"
 	"runtime/debug"
-	"syscall"
 
 	"github.com/spf13/cobra"
 	"sneak.berlin/go/keyfunc/internal/cli/age"
@@ -70,18 +67,15 @@ func Root() *cobra.Command {
 // ended with. ssh has already said whatever it had to say in that
 // case, so nothing more is printed.
 //
-// SIGINT, SIGTERM and SIGHUP cancel the command's context instead of
-// killing the process outright, so the child ssh or sftp ends and the
-// deferred cleanup that removes the agent socket and the install
-// working directory still runs.
+// SIGINT, SIGTERM and SIGHUP end the tool at once, as they end any Go
+// program, so a command waiting at the mnemonic prompt or reading what
+// it encrypts or decrypts goes no further. The exceptions catch the
+// signals to clean up first: "ssh to" and "ssh install" while they
+// have ssh or sftp running, so the child ends and their own cleanup
+// still runs, and "age encrypt -o" and "age decrypt -o" while they
+// write, so the unfinished file is removed.
 func Main() int {
-	ctx, stop := signal.NotifyContext(
-		context.Background(),
-		syscall.SIGINT, syscall.SIGTERM, syscall.SIGHUP,
-	)
-	defer stop()
-
-	err := Root().ExecuteContext(ctx)
+	err := Root().Execute()
 	if err == nil {
 		return 0
 	}