feat(plugin): add startup update check

[Statsly-org]

Summary

• Add an optional startup update checker that queries the npm registry for @st3ix/pman and logs a hint when a newer version is available.
• Default is enabled (updateCheck: true), can be disabled via plugin options.
• Non-blocking, short timeout, silent on failures/offline.

Why

Keep installations current without interrupting normal Fastify startup or Postman sync, and provide a clear npm i @st3ix/pman@latest upgrade hint.

Changes

• src/update-check.ts: background npm registry check + semver compare + log message
• src/options.ts: add updateCheck option (default true)
• src/plugin.ts: schedule the check on startup (onReady)
• README.md: document updateCheck
• test/update-check.test.js: basic coverage

Test plan

• npm test
• npm run lint
• Manual: start an app using the plugin and verify it logs pman: update available when latest > current (or stays silent otherwise).

Notes

• No secrets are written; the check uses the configured fetchImpl and never blocks startup.