Add .l-ui-page__contained--no-gutter modifier

[thomasgallagher]

Opt out of the page gutter on a contained column so it runs flush to the page edge until it reaches --l-ui-contained-width, then centres and caps. Document in the dummy app, CSS reference, and AGENTS.md.

[claude]

Review

Adds a .l-ui-page__contained--no-gutter modifier that uses negative margins to reclaim the page gutter, then re-applies centring and the width cap via padding-inline, giving a column that runs edge-to-edge on small screens and centres at --l-ui-contained-width on large ones.

No issues found. The CSS math is correct: 100% resolves to the parent's content width, so (100% - contained-width) / 2 + gutter correctly yields (viewportWidth - containedWidth) / 2 as the centering padding on wide screens, and max(0px, …) safely collapses to zero on narrow ones. The approach is consistent with how .l-ui-bleed works. Documentation is complete across the dummy app, CSS reference, and AGENTS.md.

✅ Ship