remove final --- from index.md and add guideline to agents.md

AGENTS.md

@@ -35,6 +35,7 @@ This project builds a presentation deck for Marp based on Markdown files.
 - Use ATX-style headers (# ## ###)
 - Consistent code block language identifiers
 - Frontmatter for slide metadata (title, theme, etc.)
+- Never include a final --- as it creates an unwanted empty slide
 
 ### Error Handling
 - Validate Marp syntax before build