fix(content): distinguish prose links from CTA buttons

- add reusable Link and Button MDX components
- convert source-verified page CTAs to explicit buttons
- match live-site prose link styling and CTA destinations
- document link, button, and extraction authoring rules

www/src/components/Link.astro

@@ -0,0 +1,42 @@
+---
+interface Props {
+  href: string;
+  variant?: 'default' | 'strong';
+  target?: '_blank' | '_self';
+  rel?: string;
+  class?: string;
+}
+
+const {
+  href,
+  variant = 'default',
+  target,
+  rel = target === '_blank' ? 'noopener noreferrer' : undefined,
+  class: className
+} = Astro.props;
+---
+
+<a
+  class:list={['content-link', { 'content-link--strong': variant === 'strong' }, className]}
+  href={href}
+  target={target}
+  rel={rel}
+>
+  <slot />
+</a>
+
+<style>
+  .content-link {
+    color: var(--color-accent-strong);
+    text-decoration: none;
+  }
+
+  .content-link:hover,
+  .content-link:focus-visible {
+    color: var(--color-accent);
+  }
+
+  .content-link--strong {
+    font-weight: 700;
+  }
+</style>