Clear the Storybook axe sweep's known-violations list #90

Fix the accessibility violations the Storybook axe sweep records as known.

Why

The axe sweep added in #88 checks every story in light and dark. It finds 57 serious or critical violations that already existed before the migration. They are listed in e2e/stories-a11y.known.json so the sweep can land without failing. Accessibility is a merge requirement here (see AGENTS.md), so the list should be emptied, not kept.

What is on the list

• Missing labels (stories only): the Textarea and Select stories render controls with no label (label, select-name, label-title-only).
• listitem: sidebar panel rows are <li> elements with no list around them.
• aria-required-children: ToolPicker's loading and empty states put plain text inside the listbox.
• role-img-alt: a StatusDot with label="" renders a role="img" that has no name (badge workflow status, inspector shell).
• scrollable-region-focusable: the page panel, long tool-block output and the many-steps horizontal stepper scroll but cannot be reached by keyboard.
• color-contrast: the medium RiskBadge label (--warn) measures 3.30:1 on the card in light mode.

Done when

e2e/stories-a11y.known.json is empty, or every remaining entry has a stated reason, and bun run test:e2e, check, dep-guard and contrast-test pass.