Exit 1 when a deletion prompt is not confirmed (closes #423)

`prune`, `snapshot purge`, `snapshot remove` and `database delete`
printed "Cancelled" or "Aborted." and exited 0 when the prompt was
answered no or stdin ended. They now return vaultik.ErrNotConfirmed,
so the command reports "deletion not confirmed" and exits 1.

`database delete` reads its answer from the command's stdin instead of
os.Stdin, so a test can supply it.

Model: opus-5-5

internal/vaultik/vaultik.go

@@ -5,6 +5,7 @@ package vaultik
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -184,6 +185,11 @@ func (v *Vaultik) printlnStdout(args ...any) {
 	_, _ = fmt.Fprintln(v.Stdout, args...)
 }
 
+// ErrNotConfirmed is returned when the operator answers a confirmation
+// prompt with anything but yes, or stdin ends before an answer. Nothing
+// the prompt asked about is deleted, and the command exits non-zero.
+var ErrNotConfirmed = errors.New("deletion not confirmed")
+
 // scanStdin reads a line of input from stdin.
 func (v *Vaultik) scanStdin(a ...any) (int, error) {
 	return fmt.Fscanln(v.Stdin, a...)