agents.md: Mark maps to MarkdownDocument

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -2128,13 +2128,11 @@ name = "ely-mark"
 version = "0.1.3"
 dependencies = [
  "anyhow",
- "base64 0.23.1",
  "ely-files",
  "ely-gpui-component",
  "ely-office",
  "ely-platform",
  "ely-ui",
- "ely-web-content",
  "embed-resource",
  "futures",
  "gpui-pre",