feat(astro)!: cut over production site to the astro build (develop → main release)

[dineshbyte]

Release: Astro migration cutover

Replaces the hand-authored static site on main with the Astro-generated
build that has been developed on develop. After this merges, the
main-only deploy workflow ships the Astro site to GitHub Pages — this is
the production cutover.

What ships

• The full Astro 5 site (content collections, components, token system) that
  generates docs/.
• Typography pass + type-scale tokens, simpler PR template, main-only deploy (feat(typography): type-scale tokens, simpler PR template, main-only deploy #33).
• Dark/light readability + alignment fixes across lessons and interview banks (fix(theme): dark/light readability + alignment fixes across lessons and interview banks #35).

Divergence note

main's recent #27 (analytics lesson-id) and #31 (footer note) were hot-fixes
to the old hand-authored files, which no longer exist here. Their equivalents
already live in the Astro source (#25 analytics lesson_id, #30 footer note), so
the -s ours back-merge intentionally supersedes them — nothing is lost.

Verification

• pnpm build — 98 pages, green
• node scripts/validate-content.cjs — passed
• node scripts/check-broken-links.cjs — no broken internal links
• Back-merge verified: develop tree SHA unchanged (9d64013); origin/main is
  now an ancestor, so this PR is conflict-free.