feat: sync hero snippet, highlight.js

[arjunkmrm]

Summary

• Update the landing-page hero code to mirror the README snippet (NodeContext import, platform, deepwiki + multi-line linear McpServer, Compact wrapping Messages, agent.run).
• Remap LINE_SLICE to the new line numbers so click-to-highlight still lands on the right slices.
• Swap the custom 4-token tokenizer in HeroCodeBlock for highlight.js (typescript + xml), with CSS mapping .hljs-* onto the existing keyword/string/fn/punct palette across dark and light themes.

Test plan

• pnpm dev in web/, hero renders without HMR errors; clicking the highlighted lines toggles the matching context-panel slice.