Fix for the second v0.1.1 publish failure (run 24658023641):

  npm error code MODULE_NOT_FOUND
  npm error Cannot find module 'promise-retry'
  npm error Require stack: .../arborist/lib/arborist/rebuild.js

'npm install -g npm@latest' from npm 10 to npm 11 hits a known
self-upgrade corruption bug: arborist is partially replaced
mid-install and can't find its own deps.

Cleanest workaround: pick a Node version that already ships the
npm we need. Node 24 ships npm 11.x out of the box, so we just:

- bump publish.yml to node-version: '24' and drop the fragile
  'npm install -g npm@latest' step
- add a small 'node --version / npm --version' diagnostic step
  so future CI failures are easier to triage
- bump actions/setup-node to v6 in both workflows (silences the
  'Node.js 20 actions are deprecated' annotation from setup-node@v4)

ci.yml stays on node 20 + 22 for the test matrix \u2014 that's about
which node runtimes users might run, not about which npm the
publisher needs.