refactor(link): migrate to semantic tokens and shared focus ring

[joshsadam]

Problem

Pathogen::Link styles inline navigation with raw neutral-* and primary-* utilities. Hover and text colours do not use --pvc-color-accent, and focus still uses black/white outlines.

Links appear throughout copy-heavy clinical screens, so they should match the same accent and focus rules as Button.

What we will build

Links that use semantic text and accent tokens plus --pvc-color-focus for keyboard focus.

How we will build it

• Update app/components/pathogen/link.rb LINK_CLASSES.
• Default text: --pvc-color-text or the nearest strong text role.
• Hover/accent: --pvc-color-accent / --pvc-color-accent-strong.
• Focus: focus-visible:outline-[var(--pvc-color-focus)].
• Radius: prefer --pvc-radius-action if the focus shape needs a tokenized radius.
• Check link previews and tooltip pairing still work.

Testing

• Default and hover states in light and dark mode.
• Focus ring visible and distinct from accent underline.
• External link aria-label behaviour unchanged.
• Link component tests pass.
• pnpm run build:css:check passes if class generation changes the CSS artifact.

Not in scope

• Button-styled links (Pathogen::Button with tag: :a stays separate).
• Typography inside links.
• Broad Lookbook preview cleanup.

Done when

Link matches the Foundations guidance for accent application and focus, with no hard-coded black/white outlines.

Depends on: #88
Design docs: docs/lookbook/design_system/01-design-contract.md.erb (Links, Focus And Keyboard)