Ask before removing a secret, version, vault or unlocker (closes #39)

secret rm, secret version rm, secret vault remove and secret unlocker
remove ask [y/N] on a terminal, naming what they remove, and go ahead
only on y or yes. Without --force, a command whose stdin is not a
terminal fails at once. --force, now also on rm and version rm,
removes without asking; it replaces the old refusals to remove a vault
with secrets or the last unlocker without --force. The checks run and
the question is asked before the state directory lock is taken; under
the lock the checks run again, and nothing is removed if they would
ask a different question.

Model: opus-5-5

internal/cli/cli.go

@@ -3,6 +3,7 @@ package cli
 
 import (
 	"fmt"
+	"io"
 	"os"
 
 	"git.eeqj.de/sneak/secret/internal/secret"
@@ -21,6 +22,10 @@ type Instance struct {
 	// none.
 	Mnemonic         *memguard.LockedBuffer
 	UnlockPassphrase *memguard.LockedBuffer
+	// terminal, when set, stands in for the terminal that confirm reads
+	// the user's answer from; only tests set it. When it is nil, confirm
+	// reads stdin, and only when stdin is a terminal.
+	terminal io.Reader
 }
 
 // NewCLIInstance creates a new CLI instance with the real filesystem