[Repo Assist] chore: document Node.js >=18 engine requirement in package.json

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Summary

Adds "node": ">=18" to the engines field in package.json, documenting the minimum Node.js version required by the project's test runner.

Problem

The npm run test:unit script (node --test test/unit.test.js) uses the Node.js built-in test runner, which was introduced in Node.js 18. Without an engines.node field, a developer running npm install or npm test on an older Node version (e.g. 16) gets a cryptic TypeError: The "runner" argument must be of type function. Received undefined error rather than a clear compatibility warning from npm.

Fix

Add a single field to the existing engines object:

"engines": {
    "vscode": "^1.19.0",
    "node": ">=18"
}

This is a metadata-only change — no runtime behaviour is affected.

Benefits

• npm prints a clear engine compatibility warning if developers have a mismatched Node version
• Documents the actual runtime requirement (already implicitly required by test:unit)
• Appears in the VS Code Marketplace extension manifest for transparency

Test Status

✅ All 7 unit tests pass: npm run test:unit
✅ Syntax check clean: node --check extension.js

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Warning

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This was originally intended as a pull request, but the patch modifies protected files: package.json.

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