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

prune, snapshot purge, snapshot remove and database delete exited 0 when their confirmation prompt was declined or left unanswered, so a script could not tell that nothing was deleted. They now end with "deletion not confirmed" and exit 1. The [y/N] prompts accept y or Y; database delete still asks for yes.

Model: opus-5-5

internal/vaultik/snapshot_list_test.go

@@ -362,8 +362,9 @@ func TestSnapshotListAndPurgePrintSizesLikeRemoteInfo(t *testing.T) {
 	// Without --force and with nothing on stdin the purge lists what it
 	// would delete, reads no confirmation, and deletes nothing.
 	env.stdout.Reset()
-	require.NoError(t, env.v.PurgeSnapshotsWithOptions(
-		&vaultik.SnapshotPurgeOptions{OlderThan: "1d"}))
+	require.ErrorIs(t, env.v.PurgeSnapshotsWithOptions(
+		&vaultik.SnapshotPurgeOptions{OlderThan: "1d"}),
+		vaultik.ErrNotConfirmed)
 	assert.Contains(t, findTableRow(t, env.stdout.String(), listLocalID),
 		wantSize, "snapshot purge")
 }