chore: update dev dependency pins and fix README build instructions

[devin-ai-integration]

Refreshes the exact dev-dependency pins to the latest within their current major, and fixes README build instructions that told users to edit a variable that no longer exists.

• @types/node 22.10.1 → 22.20.1, typescript 5.7.2 → 5.9.3 (exact pins, same major); ts-node already latest (10.9.2).
• LaunchDarkly SDK constraint left alone: ">= 9.0.0" already resolves to the latest 9.x (9.13.1) and no lock file is committed, so a floor bump would be churn.
• README step 2 now documents LAUNCHDARKLY_SDK_KEY / LAUNCHDARKLY_FLAG_KEY env vars — index.ts has no editable sdkKey = "1234..." literal anymore; wording matches hello-node-server.
• Dev-dependency + docs only, hence chore: — no shipped code or user-facing requirement changes.

Implementation details

Verified locally on Node 24.16.0 with the updated pins:

• npm i → @launchdarkly/node-server-sdk@9.13.1, @types/node@22.20.1, typescript@5.9.3; 0 vulnerabilities.
• npx tsc --noEmit index.ts → clean, no type errors and no deprecation warnings.
• CI=1 LAUNCHDARKLY_SDK_KEY=… LAUNCHDARKLY_FLAG_KEY=hello-boolean npm start:

info: [LaunchDarkly] Opened LaunchDarkly stream connection
*** SDK successfully initialized!
*** The 'hello-boolean' feature flag evaluates to true.

No deprecated LaunchDarkly SDK APIs are used (init, waitForInitialization({timeout}), variation, on('update:…') are all current). Console app, so no screenshots.

Note: the "Require additional human oversight on bot PRs" check stays red by org policy and needs a human.

Link to Devin session: https://app.devin.ai/sessions/73b046543ab7426180689a07de610afa

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Note

Overview
README build steps no longer tell users to edit sdkKey / featureFlagKey in index.ts. Step 2 now documents LAUNCHDARKLY_SDK_KEY and LAUNCHDARKLY_FLAG_KEY (with bash export examples), notes the default flag key sample-feature, and adds what to expect after npm start (evaluation message and live flag updates).

Dev dependencies are bumped on the same majors: @types/node 22.10.1 → 22.20.1 and typescript 5.7.2 → 5.9.3. Runtime dependency on @launchdarkly/node-server-sdk is unchanged.

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[devin-ai-integration]

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[devin-ai-integration]

@cursor review