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

@@ -185,10 +185,9 @@ func (v *Vaultik) printlnStdout(args ...any) {
 	_, _ = fmt.Fprintln(v.Stdout, args...)
 }
 
-// ErrNotConfirmed is returned when a confirmation prompt gets any answer
+// ErrNotConfirmed is returned when the operator answers a confirmation
-// but the one it asks for (`y` or `Y`, or `yes` for `database delete`), or
+// prompt with anything but yes, or stdin ends before an answer. Nothing
-// stdin ends first. Nothing the prompt asked about is deleted, and the
+// the prompt asked about is deleted, and the command exits non-zero.
-// command exits non-zero.
 var ErrNotConfirmed = errors.New("deletion not confirmed")
 
 // scanStdin reads a line of input from stdin.