Add an agent-facing llms.txt directive to page HTML

[chalin]

• Contributes to Improve support for AI-agent doc consumption #2614 (phase 2 opener).
• Emits a hidden llms.txt directive at the top of every page body, the primary discovery mechanism agents use to find a site's Markdown path (afdocs: the directive reaches all agents, unlike content negotiation).
• Free for every Docsy site: gated on the site already publishing llms.txt (the LLMS output format), no new configuration; non-opted sites render byte-identically. Sites overriding the theme's baseof templates must call the new partial themselves (documented on the agent-support page).
• docsy.dev adoption: drops the fr home's stale outputs override (it masked the site config's LLMS output, so fr pages emitted no directive) and refreshes the published scorecard: 88 (B) -> 96 (A). Two causes: the directive check passes on all sampled pages, and content-start-position flips to PASS because the checker counts the directive itself as first content; the underlying nav-before-content DOM order is unchanged and stays tracked under Improve support for AI-agent doc consumption #2614.
• Preview:
  • Agent support (page docs + embedded scorecard)
  • Docs landing: view-source for the directive