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

SIGINT, SIGTERM and SIGHUP were caught for the whole run, so the
mnemonic prompt waited for Enter and an interrupted `age encrypt -o`
put the encryption of the cut-off input in place. Now they end the
tool at once, except where a command cleans up first: `ssh to` and
`ssh install` while ssh or sftp runs, and `age encrypt -o` and
`age decrypt -o` while they write. A signal those two have received
when their input ends removes the unfinished file and exits 1; a later
one leaves the whole file in place. The tool stays on the main thread,
where Linux delivers the signal first. Signals the tool was started
ignoring stay ignored, so a run under nohup survives a hangup.

Model: opus-5-5

internal/cli/ssh/to.go

@@ -10,6 +10,7 @@ import (
 	"syscall"
 
 	"github.com/spf13/cobra"
+	"sneak.berlin/go/keyfunc/internal/cli/signals"
 )
 
 // StatusError says the tool should end with the status ssh ended with.
@@ -42,7 +43,14 @@ 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 := signals.Context(cmd.Context())
+			defer stop()
+
+			served, err := key.Serve(ctx, comment)
 			if err != nil {
 				return err
 			}
@@ -53,7 +61,7 @@ func to() *cobra.Command {
 				"-o", "IdentityAgent=" + served.Socket(),
 			}, args)
 
-			return connect(cmd.Context(), argv)
+			return connect(ctx, argv)
 		},
 	}
 
@@ -76,7 +84,7 @@ 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 {