Add muted hover card footer

[brsbl]

What changed

The hover-card repository metadata now lives in a semantic footer component. The footer uses the existing --muted token at 60% opacity with full-strength --muted-foreground text, a transparent-to-muted transition, and compact 0.25rem top padding.

Dark mode adds a restrained internal gold sheen and warm border using the existing warning tokens. It retains normal card elevation but deliberately adds no emitted gold glow.

The footer content and interactions are unchanged.

Why

The previous footer labels and PR metadata competed visually with the card body, and a text-only adjustment was too subtle. The dedicated footer and dark-mode surface improve hierarchy while remaining theme-aware.

Validation

• npm run check --workspace=bb-plugin-thread-hover-cards
• DOM coverage for semantic footer grouping, local and git variants, 60%-opacity muted styling, fade, compact padding, dark-mode token styling, and absence of an outer gold glow
• Installed local bundle matched the checked source artifact and reloaded with running status
• Nightly visual verification attempted; its automated sidebar state did not open the hover card, so no substitute screenshot from the regular bb app is attached