Fix duplicate ids, validate CLI input, harden load/save

- add() uses max id + 1 so ids are never reused after rm
- done/rm reject missing, non-numeric and unknown ids; add rejects empty text
- load() gives a readable error on bad JSON or non-list data
- save() writes a temp file and renames it
- CLI checks the command before loading and no longer relies on process.exit for ls
- Add regression tests for all of the above

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/cli.js

@@ -1,28 +1,45 @@
 #!/usr/bin/env node
-import { load, save, add, complete, remove, format } from "./todo.js";
+import { load, save, add, has, complete, remove, format } from "./todo.js";
 
 const FILE = process.env.TODO_FILE ?? "todos.json";
-const [cmd, ...args] = process.argv.slice(2);
-let todos = load(FILE);
+const USAGE = "Usage: todo [ls | add <text> | done <id> | rm <id>]";
+const [cmd = "ls", ...args] = process.argv.slice(2);
 
-switch (cmd) {
-  case "add":
-    todos = add(todos, args.join(" "));
-    break;
-  case "done":
-    todos = complete(todos, Number(args[0]));
-    break;
-  case "rm":
-    todos = remove(todos, Number(args[0]));
-    break;
-  case "ls":
-  case undefined:
-    console.log(format(todos));
-    process.exit(0);
-  default:
-    console.error(`Unknown command: ${cmd}\nUsage: todo [ls | add <text> | done <id> | rm <id>]`);
-    process.exit(1);
+function fail(message) {
+  console.error(message);
+  process.exit(1);
 }
 
-save(FILE, todos);
+function parseId(todos, arg) {
+  const id = Number(arg);
+  if (arg === undefined || !Number.isInteger(id)) fail(`Invalid id: ${arg ?? "(missing)"}\n${USAGE}`);
+  if (!has(todos, id)) fail(`No todo with id ${id}`);
+  return id;
+}
+
+if (!["ls", "add", "done", "rm"].includes(cmd)) fail(`Unknown command: ${cmd}\n${USAGE}`);
+
+let todos;
+try {
+  todos = load(FILE);
+} catch (err) {
+  fail(err.message);
+}
+
+switch (cmd) {
+  case "add": {
+    const text = args.join(" ").trim();
+    if (!text) fail(`Missing todo text\n${USAGE}`);
+    todos = add(todos, text);
+    break;
+  }
+  case "done":
+    todos = complete(todos, parseId(todos, args[0]));
+    break;
+  case "rm":
+    todos = remove(todos, parseId(todos, args[0]));
+    break;
+}
+
+if (cmd !== "ls") save(FILE, todos);
 console.log(format(todos));