feat(callout): composable/headless Callout — neutral variant + custom content

[bntvllnt]

Problem

Callout is opinionated in a way that blocks reuse for editorial/branded
call-outs. It (a) forces the internal structure — an optional icon slot, a title
<p class="font-semibold">, and a body wrapped in text-sm [&>p]:mb-0; and (b)
its variants are all colored surfaces (danger/info/note/success/tip/
warning, each a bg-*/10 border-* + fixed text color). A consumer that wants a
neutral or brand-accented call-out — e.g. a left-rule aside driven by --accent,
no colored background, with its own header + body typography — can't get there
without overriding half the classes and still fighting the fixed inner markup.

Proposal

• A neutral, token-driven variant (no colored bg/border/text; border + text
  from the standard tokens, so a brand accent comes from --accent / className).
• Composable/headless usage: let the header and body be arbitrary children
  (e.g. CalloutHeader / CalloutBody subcomponents, or accept title as a
  ReactNode and drop the forced font-semibold / text-sm), so consumers own
  the inner typography and layout while the component keeps the container role,
  spacing and border.

Acceptance

• A neutral/token-driven variant with no colored surface
• Consumer can supply a custom header + body without the forced font-semibold/text-sm
• Container role/spacing/border still provided; brand accent via token/className
• Existing colored variants unchanged (backward compatible)

[github-actions]

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