Enforce integrity validation in CI

[GsCommand]

Motivation

• Ensure the CI validation workflow enforces the full release-critical path so schema, example, checksum, and integrity drift are detected on pushes and pull requests and a PR cannot merge with integrity regression.

Description

• Add npm run validate:integrity to .github/workflows/validate.yml before checksum generation and adjust the checksum diff invocation to git diff --exit-code -- checksums.txt to avoid path ambiguity.

Testing

• Ran npm run validate:schemas, npm run validate:examples, npm run validate:integrity, and npm run generate:checksums && git diff --exit-code -- checksums.txt locally and all commands completed successfully.

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Codex Task