ci: Run changelog validation in a single job

[Mrtenz]

Explanation

Instead of running 83 jobs for changelog validation, we can run it in a single job, which should not affect the total CI time much.

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Low risk CI-only change; main risk is reduced parallelism and potential failures if the loop/jq parsing behaves differently than the previous matrix expansion.

Overview
Reworks the lint-build-test.yml changelog validation to run in one validate-changelogs job instead of spawning a matrix job per workspace.

The new step reads the child-workspace-package-names JSON output and loops through each package, running yarn workspace <pkg> changelog:validate while aggregating failures into a single exit code.

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