Add CSS skill and docs

What does this MR do?

Adds a CSS section to Pajamas documenting how to style consistently with the design system, and a css agent skill pointing at it for the same guidance for human and AI authors.

Foundations documents which value to use. Nothing documented how to apply it: which property, which utility, and the ways a plausible-looking class attribute silently does nothing. That lived in review comments.

Two themes

Correctness — every declaration should have an effect:

• Clashing properties: gl-sr-only gl-font-bold styles text no one sees.
• Clashing values: gl-m-4 gl-m-0 — the compiled stylesheet decides the winner, not the attribute order.
• Utilities that can't apply: gl-gap-* without flex, gl-border-2 without a border style, z-index without positioning.

Scalability — styles should survive change:

• Semantic tokens over constants, so a style follows a color mode change or design system migration.
• Container-owned spacing, min-width-first responsive styles, and low specificity, so a component can be moved or refactored without unpicking its styles.

What's added

Seven pages under a new CSS nav section — Principles, Spacing, Layout, Typography, Color, Border, Elevation. Principles carries the cross-cutting rules, the Tailwind utility and design token guidance, and a review checklist; each area page applies them to a group of properties and ends with Common pitfalls.

Every rule is a do/don't pair of live-example blocks: rendered result above editable source, so the failure is visible rather than described. Foundations pages gain a Code reference link to their CSS counterpart.

The css skill (.claude/skills/css/SKILL.md, plus a line in AGENTS.md) reads these pages over the public raw endpoint rather than duplicating them, and states the two rules agents most often get wrong: judge against the guidance, not surrounding code, and verify class names against the preset instead of inventing them.

Screenshots or screen recordings

n/a

Integrations

n/a

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

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Conformity

• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• Added the ~"component:*" label(s) if applicable.
• A changeset is added when this MR will do a patch, minor, or major update. More information can be found in doc/changesets.md.

Components

• GitLab UI's contributing guidelines.
• If the MR changes a component's API, integration MR(s) have been opened (see integrations above).

Documentation

• If creating a new component page from scratch, it follows the page template structure.
• Content follows the GitLab Documentation Style Guide when appropriate.
• Related pages are cross-linked, where helpful. Component pages have related components and patterns defined in their Markdown front matter.
• If embedding a Figma file, it follows the Figma embed guide.
• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.
• Maintainer review follows the Pajamas UX maintainer review checklist.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.