Document that --json skips the confirmation prompt (closes #268)

snapshot remove and prune delete without asking under --json, because a
prompt on stdout would break the JSON document. Their --json help and
README entries named only --force as skipping the prompt. Both now say
--json skips it too. A test checks the --json help of both commands.

Model: opus-5-5

internal/cli/prune.go

@@ -57,7 +57,8 @@ referenced.`,
 	}
 
 	cmd.Flags().BoolVar(&opts.Force, "force", false, "Skip confirmation prompt")
-	cmd.Flags().BoolVar(&opts.JSON, "json", false, "Output pruning stats as JSON")
+	cmd.Flags().BoolVar(&opts.JSON, "json", false,
+		"Output pruning stats as JSON; skips the confirmation prompt, as --force does")
 
 	return cmd
 }