fix(infra): agent-friendly docs spec v0.5.0 — easy compliance wins (#182)

Closes two of the six failures found in #181.

## Summary

- **`llms-txt-directive-md`**: inject `> For the complete documentation
index, see [llms.txt](/llms.txt)` near the top of every generated `.md`
file, inside `cleanMarkdown()` in `plugins/astro-agent-docs.mjs`. This
is a new mandatory requirement in spec v0.5.0 (split from the previous
single `llms-txt-directive` check).
- **`content-negotiation`**: add `public/_headers` to serve `.md` files
with `Content-Type: text/markdown; charset=utf-8`. Fixing this may also
resolve the `content-start-position` failures (34/50 pages) as a side
effect — once agents receive a proper `text/markdown` response, they
should use the clean `.md` files (which start with content immediately)
instead of converting Starlight's nav-heavy HTML.

## Sync recommendation

hand-written

Author: marc0olo

plugins/astro-agent-docs.mjs

@@ -87,6 +87,9 @@ const SECTIONS = deriveSections(sidebar);
 // a charset=utf-8 in the Content-Type header.
 const BOM = "\uFEFF";
 
+const LLMS_TXT_DIRECTIVE =
+  "> For the complete documentation index, see [llms.txt](/llms.txt)\n\n";
+
 /** Strip YAML frontmatter, HTML comments, and MDX artifacts; prepend title heading. */
 function cleanMarkdown(raw, isMdx = false) {
   const { data, content } = matter(raw);
@@ -96,7 +99,7 @@ function cleanMarkdown(raw, isMdx = false) {
   }
   body = body.replace(/\n{3,}/g, "\n\n").trim();
   const title = data.title ? `# ${data.title}\n\n` : "";
-  return BOM + title + body + "\n";
+  return BOM + title + LLMS_TXT_DIRECTIVE + body + "\n";
 }
 
 /**

public/_headers

@@ -0,0 +1,2 @@
+/*.md
+  Content-Type: text/markdown; charset=utf-8