Address review round 2: duplicate ids, read-only files, dangling links, safe ids

- load() rejects duplicate ids (left by the old add bug) and ids that aren't safe positive integers
- save() refuses files the user can't write, and follows dangling symlinks to their target
- CLI rejects ids beyond Number.MAX_SAFE_INTEGER
- Skip permission-based tests when running as root

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

test/cli.test.js

@@ -41,6 +41,7 @@ for (const [label, args, message] of [
   ["decimal id", ["1.0"], /Invalid id: 1\.0/],
   ["padded id", [" 1 "], /Invalid id:  1 /],
   ["empty id", [""], /Invalid id: \n/],
+  ["unsafe id", ["99999999999999999999"], /Invalid id: 99999999999999999999/],
   ["unknown id", ["99"], /No todo with id 99/],
 ]) {
   for (const cmd of ["done", "rm"]) {
@@ -64,7 +65,7 @@ test("unknown command prints usage even when the file is corrupt", () => {
   assert.match(res.stderr, /Unknown command: --help\nUsage:/);
 });
 
-test("a failed save gives a readable error instead of a stack trace", () => {
+test("a failed save gives a readable error instead of a stack trace", { skip: process.getuid?.() === 0 && "root ignores file permissions" }, () => {
   const file = tempFile();
   chmodSync(dirname(file), 0o500);
   try {