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/to.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"os/signal"
 	"slices"
 	"syscall"
 
@@ -42,7 +43,17 @@ func to() *cobra.Command {
 				return err
 			}
 
-			served, err := key.Serve(cmd.Context(), comment)
+			// From here until the agent is taken down, a signal
+			// cancels the context instead of ending the tool, so
+			// ssh ends and the socket and its directory are still
+			// removed.
+			ctx, stop := signal.NotifyContext(
+				cmd.Context(),
+				syscall.SIGINT, syscall.SIGTERM, syscall.SIGHUP,
+			)
+			defer stop()
+
+			served, err := key.Serve(ctx, comment)
 			if err != nil {
 				return err
 			}
@@ -53,7 +64,7 @@ func to() *cobra.Command {
 				"-o", "IdentityAgent=" + served.Socket(),
 			}, args)
 
-			return connect(cmd.Context(), argv)
+			return connect(ctx, argv)
 		},
 	}
 
@@ -76,7 +87,7 @@ func connect(ctx context.Context, argv []string) error {
 	command.Stdout = os.Stdout
 	command.Stderr = os.Stderr
 
-	// A cancelled context means a signal ended the tool. Send ssh a
+	// A cancelled context means a signal arrived. Send ssh a
 	// SIGTERM rather than the default kill, so it puts the terminal
 	// back the way it found it before it goes.
 	command.Cancel = func() error {