feat: depreceate scroll-area component and styled scrollbars

[Shrinks99]

These are left over from the component library conversion and not elements that I want to have stick around! Deprecating in favour of browser-native scroll controls (which we use almost everywhere anyways). Now that we have a skill which gives agents a lot of context on our components, it's becoming more important to clean this stuff up so it doesn't get implemented more widely.

Also adds a changelog going forward for the component library.

[tmccoy14]

Reviewed the scroll-area deprecation across all 3 commits. The removal itself is clean and complete — both CSS entry points updated, no lingering references to the deleted utilities in source or built CSS, and Dialog/Sheet still scroll correctly since .dialog-body and .sheet-body already carry overflow-auto. The docs plumbing works too: llms.txt and the .md twins render the deprecation as intended. lint, format:check, and the full build all pass.

Three things to fix in the CHANGELOG before merging, noted inline.

[tmccoy14]

This claims a change that isn't in the branch. grep -rni "scrollbar|scroll area|overflow" packages/skills/ returns nothing — the equality-design-system skill is untouched.

This is the gap CLAUDE.md specifically calls out: the skill is what stops agents in other repos rebuilding what we already ship. Without it, an agent consuming the skill still has no rule against hand-rolling ::-webkit-scrollbar styling. The "Common Mistakes" list in packages/skills/equality-design-system/SKILL.md is the natural home for it.

Either add the guidance to the skill, or drop this line.

[tmccoy14]

DialogBody and SheetBody aren't exported — there are no such names anywhere in packages/ui/src. The components actually changed are DialogContent and SheetContent (they consume the dialog-body / sheet-body CSS module classes, which is probably where the naming came from).

[tmccoy14]

"Explorer" should be "Equality". Worth catching since this file was added to files in package.json, so it now ships to npm and renders on the package page.

[tmccoy14]

Heads up on downstream impact: guardian/apps/studio still has live <ScrollArea /> usage in five files.

• src/components/indicators/apply-indicator-to-control.tsx:209
• src/components/indicators/indicator-search-list.tsx:90
• src/components/policies/apply-policy-dialog.tsx:343
• src/components/controls/control-review-sidebar.tsx:104
• src/components/controls/shimmers/ControlViewerShimmer.tsx:154

Nothing breaks on merge — studio pins "@eqtylab/equality": "4.0.0" exactly, and it has no usages of the removed styled-vertical-scrollbar / styled-horizontal-scrollbar classes, so the utility removal is a no-op there. The @deprecated JSDoc will start showing up as strikethrough in the editor once someone bumps the pin.

Three of the five are the straightforward h-* + overflow-y-auto swap from the migration guide. Two need a closer look:

• apply-policy-dialog.tsx:343 uses w-[calc(100%+16px)] pr-4 — that's compensating for the Radix scrollbar's gutter, so the negative-margin trick should come out along with the component rather than be carried over.
• apply-indicator-to-control.tsx:209 has a bare <ScrollArea> with no height constraint, so it's relying on a flex parent to bound it. Worth confirming it still clips once the Radix viewport wrapper is gone.

Not a blocker for this PR — just flagging so the studio migration gets tracked rather than discovered at the next version bump.