fix(agent-docs): include hero title in markdown endpoint for parity

[marc0olo]

Closes part of #181 (markdown-content-parity).

Summary

• Starlight splash pages render hero.title (and hero.tagline if present) in the HTML, but these fields live in frontmatter and are not part of the content body — so they were absent from the generated .md endpoint.
• The cleanMarkdown() function in plugins/astro-agent-docs.mjs now extracts hero.title and hero.tagline and inserts them between the page title heading and the llms.txt directive, matching what agents see in the HTML.
• Verified locally: dist/index.md now starts with # Build on the Internet Computer → The Network is The Cloud. Agents Build. → llms.txt directive → body content.
• Build passes cleanly.

Sync recommendation

hand-written