CC-07: Add tabbed goal navigation to the Getting Started hero

Source document: .agents/core-content-promotion-plan.md (§3.1 “Modular hero”)
Owner: 🤖 Agent
Priority: Medium
Status: Not started. getting-started.md has a hero with anchor jump links to #step-1 to #step-4, but no tabs that preview the key articles.
Depends on: None

Task

Let visitors preview the key articles for each of the four goals inside the hero, through an accessible tab interface, without scrolling.

File: getting-started.md (layout: default, custom .gs-* markup). Styles are in _sass/_getting-started.scss.
Steps and goals on the page: step-1 “Learn the rules and rhythm of play”, step-2 “Pick your first set of bowls”, step-3 “Build core skills and repeatable delivery”, step-4 “Play your first competitive games”.
Use the WAI-ARIA Tabs pattern (role="tablist"/tab/tabpanel, arrow-key navigation, aria-selected, roving tabindex). Without JS, the content must stay readable (progressive enhancement: the anchor links still work).
JS goes in a module under assets/js/ (e.g., assets/js/tabs.js), loaded with defer. No inline scripts. Add tests if a JS test setup exists (npm test).
Can reuse the action cards from CC-08 inside each panel.

Four tabs, one per goal, each previewing two or three key articles with links.
Meets the ARIA tabs pattern (keyboard arrows, Home/End, focus management); works without JS.
Lighthouse accessibility ≥ 90 on /getting-started/.
bundle exec jekyll build completes without errors.