Print a failed command's error once on stderr (closes #433)

A failed command printed its error twice on stderr: as an ERROR log record and as the final ERROR: line the README documents. The command handlers, a restore failure that ends the command and a failed blob upload now log that error at info level or not at all, so without --verbose or --debug only the final line appears.

Model: opus-5-5

internal/cli/prune.go

@@ -60,7 +60,7 @@ referenced.`,
 			}, func(v *vaultik.Vaultik) error {
 				return v.Prune(opts)
 			}, func(err error) {
-				log.Error("Prune operation failed", "error", err)
+				log.Info("Prune operation failed", "error", err)
 				ReportErrorf("Prune failed: %v", err)
 			})
 		},