Harden CI: run on Node 20 & 22 and add entrypoint/pack smoke tests

[GsCommand]

Motivation

• Improve CI hardening by verifying the package on recent LTS Node versions (20 and 22) and adding lightweight runtime checks that surface obvious packaging or import-time issues without changing schemas or package structure.
• Ensure existing validation and checksum verification steps remain intact and are exercised in CI to prevent regressions.

Description

• Update the workflow at .github/workflows/validate.yml to use a matrix node-version: [20, 22], enable cache: npm, and switch installs to npm ci, while preserving the existing validation steps and adding smoke-test steps for import and pack.
• Add scripts/smoke-import.mjs which imports the package entrypoint and asserts exports exist, and add scripts/smoke-pack.mjs which runs npm pack --json into a temp dir, tolerates mixed stdout from prepack, and verifies a non-empty tarball is produced.
• Add two npm scripts in package.json: test:smoke:import and test:smoke:pack, and keep existing validate and checksum scripts unchanged.
• No schemas, example data, package exports, publish behavior, or docs were altered. The smoke pack script only parses npm pack --json output more defensively to handle prepack stdout lines.

Testing

• Ran npm run validate which executed npm run validate:schemas and npm run validate:examples and completed successfully (all schemas compiled and examples validated).
• Ran npm run checksums:verify which verified checksums for all listed schema files and succeeded.
• Ran npm run test:smoke:import which imported index.js and passed, reporting 20 exports.
• Ran npm run test:smoke:pack which produced an npm pack tarball and verified it is present and non-empty.
• Attempted a local Node 20 run via npx -p node@20 but it was blocked by registry policy (npm returned 403), so Node 20 could not be validated locally here; the workflow matrix will validate Node 20 and 22 in CI.

Files changed:

• .github/workflows/validate.yml
• package.json
• scripts/smoke-import.mjs
• scripts/smoke-pack.mjs

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Codex Task