Fix /education homepage cards to deep-link into scoped /lessons views

[jt14den]

Part of #194.

Current state

src/pages/index.astro (lines 36-57), the 3 homepage route cards:

• "Browse all lessons" → /lessons (unfiltered)
• "New to open source?" → /lessons#getting-started-heading (anchor scroll only, not a filtered state)
• "Planning instruction?" → /lessons (unfiltered — identical target to "Browse all lessons")

Feedback (2026-08-04 WG meeting)

All 3 cards effectively land on the same unscoped page. Each card should land on a genuinely scoped/filtered /lessons state matching its stated intent, not just an anchor jump or the bare catalog.

Depends on

Getting-Started-as-facet issue (for what "New to open source" links to), filter-simplification issue (for what a shareable filtered-URL state looks like)

Related

#194