fix(cli): warn when postinstall auto-update sync fails

[ZayanKhan-12]

Description

In postinstall() in bin/metamask-skills.mjs, the exit code from the auto-update sync delegate was discarded by a ternary that returns 0 on both branches:

const result = delegate('sync', target, repo, passthrough);
return result === 0 ? 0 : 0;

So when the delegated sync script exits non-zero (e.g. the bash delegate fails, or spawnSync can't start it and delegate returns 1), the failure is silently ignored — while thrown errors in the same path do emit a warning via the catch block.

This change keeps the intentional never-fail contract of postinstall (a failed auto-update should not break npm install, so it still returns 0), but surfaces the failure the same way the catch path does:

const result = delegate('sync', target, repo, passthrough);
if (result !== 0) {
  warn(`auto-update failed with exit code ${result}`);
}
return 0;

Testing

• node --test test/*.test.mjs — 36/36 pass
• node bin/metamask-skills.mjs --help smoke check passes

🤖 Generated with Claude Code