The ssh install and ssh to commands (closes #2)

install runs the system ssh and hands the host a short shell script.
The public key line reaches it on standard input, never on a command
line others on the host could read. The script makes ~/.ssh and
authorized_keys if missing, adds the line unless it is already there,
and says which of the two it did.

to serves the key from an agent inside the tool, on a unix socket in a
temporary directory only its owner can enter, and points ssh at it with
-o IdentityAgent. Socket and directory go when the command ends and the
private key is never written to disk. Only this command ends with the
status ssh ended with rather than status 1.

Model: opus-5

internal/cli/cli.go

@@ -2,6 +2,7 @@
 package cli
 
 import (
+	"errors"
 	"fmt"
 	"os"
 
@@ -36,14 +37,22 @@ func Root() *cobra.Command {
 }
 
 // Main runs the tool and returns the status the process should exit
-// with.
+// with. An error ends the tool with status 1, except when it carries a
+// status of its own, which "ssh to" uses to hand on the status ssh
+// ended with. ssh has already said whatever it had to say in that
+// case, so nothing more is printed.
 func Main() int {
 	err := Root().Execute()
-	if err != nil {
-		fmt.Fprintln(os.Stderr, "keyfunc: "+err.Error())
-
-		return 1
+	if err == nil {
+		return 0
 	}
 
-	return 0
+	var passed ssh.StatusError
+	if errors.As(err, &passed) {
+		return passed.Status
+	}
+
+	fmt.Fprintln(os.Stderr, "keyfunc: "+err.Error())
+
+	return 1
 }