Add `BareButton` design-system primitive

[talissoncosta]

When we need a clickable surface that doesn't look like a button (stepper steps, selectable cards, list rows), the current options are both bad:

• <div role='button' tabIndex={0} onKeyDown={…}> — manual keyboard handling, easy to get wrong, no disabled semantics
• <button> + per-component all: unset or manual resets — repeats the reset everywhere, easy to drift

A BareButton primitive would give us all the semantics and accessibility of a native button (focus, disabled, keyboard, screen-reader role) with no visual styling, so it can wrap any clickable surface.

Scope

• New component, likely under web/components/base/forms/BareButton/
• Renders <button type='button'> with browser defaults reset (all: unset + minimal layout primitives)
• Forwards refs, accepts all <button> props
• Storybook story + Jest test

Migration targets (follow-up PRs)

• WizardStepper step rows (PR feat(experimentation): add experiment creation wizard frontend #7596) — currently <div role='button'>
• SelectableCard (merged in PR feat(experimentation): rework warehouse connection UI #7581) — currently <div role='button'>
• Any other <div role='button'> sites surfaced by a grep

Context

Came up on PR #7596 and earlier on #7581 — same pattern twice in two weeks. Cheaper to extract a primitive than fix each instance with manual all: unset.