Clean up the agent socket and working files when a signal ends the tool (closes #17)

`cli.Main` ran the command tree on a background context, so SIGINT, SIGTERM or SIGHUP killed the process before deferred cleanup ran: `ssh to` left its agent socket and directory behind, and `ssh install` left a copy of the host's `authorized_keys` in its working directory. `Main` now runs the tree on a `signal.NotifyContext` for those signals; the cancelled context ends the child `ssh` or `sftp` and the cleanup runs. `ssh to` stops its child with SIGTERM, not a kill, so `ssh` restores the terminal. Exit status after a signal is 1 unless `ssh` reported its own.

The test re-runs the test binary as the tool, waits for the agent socket, sends each signal and checks the directory is gone.

Disclosure: the repeated `"uptime"` test literal became a `remoteCommand` constant because `goconst` required it.

Model: opus-4-8 (implementation, review); fable-5-1 (merge message)

internal/cli/ssh/to.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"os/exec"
 	"slices"
+	"syscall"
 
 	"github.com/spf13/cobra"
 )
@@ -75,6 +76,13 @@ 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
+	// SIGTERM rather than the default kill, so it puts the terminal
+	// back the way it found it before it goes.
+	command.Cancel = func() error {
+		return command.Process.Signal(syscall.SIGTERM)
+	}
+
 	err := command.Run()
 	if err == nil {
 		return nil