fix: align README with env-var setup and restore missing client-side ID guard

[devin-ai-integration]

Summary

Routine SDK audit of this example. launchdarkly-node-client-sdk needed no version change: the manifest declares >= 3.4.0, latest on npm is 3.4.3, and there is no committed lock file, so npm install already resolves to the latest 3.x. No deprecated SDK APIs are used (initialize, waitForInitialization(timeout), variation, on('change:...') are all current; no deprecation warnings on install or run).

Two real defects found instead:

1. README was stale — it told users to edit index.js and set a variable environmentId that does not exist (the code reads LAUNCHDARKLY_CLIENT_SIDE_ID / LAUNCHDARKLY_FLAG_KEY), and quoted an output line the app no longer prints. Rewrote the build instructions in the same shape as hello-node-server.

2. The "please set your client-side ID" guard was dead code:

-const clientSideId = process.env.LAUNCHDARKLY_CLIENT_SIDE_ID ?? 'your-client-side-id';
+const clientSideId = process.env.LAUNCHDARKLY_CLIENT_SIDE_ID ?? '';
 ...
 if (!clientSideId) { /* never reached before: the placeholder is truthy */ }

Without the env var the example previously tried to initialize with the literal your-client-side-id and failed with an opaque SDK error instead of the intended message.

Titled fix: rather than chore: because it touches shipped example code.

Validation

$ npm install                      # installs launchdarkly-node-client-sdk 3.4.3, 0 vulnerabilities
$ LAUNCHDARKLY_CLIENT_SIDE_ID=<client-side id> CI=1 node index.js
info: [LaunchDarkly] LaunchDarkly client initialized
*** SDK successfully initialized!
*** The 'sample-feature' feature flag evaluates to false.

$ node index.js                    # no env var
*** Please set LAUNCHDARKLY_CLIENT_SIDE_ID to your client-side ID, or edit index.js to use your client-side ID.
(exit 1)

Limitations

Node console app, so no screenshots. Repo CI (verify-hello-app) skips markdown-only changes but runs for the index.js change.

Link to Devin session: https://app.devin.ai/sessions/a4f2f04fc40f4596aa5f7dd97ef1a223

[devin-ai-integration]

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[devin-ai-integration]

@cursor review