Build/Test Tools: Add Stylelint checks to CSS precommit

[Sukhendu2002]

This PR adds Stylelint checks to the CSS pre-commit workflow, as discussed in Trac ticket https://core.trac.wordpress.org/ticket/29792

The new lint:css task checks core CSS for:

• Empty blocks.
• Unknown properties.
• Units on zero-length values.
  The task runs before postcss:core as part of precommit:css. Generated minified and RTL files are excluded.

Testing

• Verified the pre-commit workflow locally
• Confirmed Stylelint runs against the expected CSS files
• Tested the changes with both valid CSS and CSS containing linting errors

Use of AI Tools

AI assistance: Yes
Tool(s): Pi
Model(s): GPT-5.6
Used for: Initial code skeleton, code review and PR description generation; final implementation and tests were reviewed and edited by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[github-actions]

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Props sukhendu2002.

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