add critical file protection rules for index.md

CLAUDE.md

@@ -6,6 +6,13 @@ This project builds a presentation deck for Marp based on Markdown files.
 - Agent NEVER runs commands on its own except changing files INSIDE THIS FOLDER, never any other then this
 - Agent NEVER runs build commands or any automated processes without explicit user request, as the user is working in dev mode
 
+## Critical File Protection: index.md
+- `index.md` is the MAIN CONTENT FILE for the entire lecture series - treat with extreme care
+- ALLOWED: Adding new slides, adjusting existing content, fixing typos, enhancing sections
+- FORBIDDEN (without explicit permission): Deleting slides, removing sections, bulk replacements that remove content
+- Before ANY deletion in index.md: ALWAYS ask user for confirmation and warn about what will be removed
+- When in doubt, ADD rather than REPLACE - let the user decide what to remove
+
 ## Build Commands
 - `npm run build` - Build slides from Markdown using Marp
 - `npm run dev` - Start development server at http://localhost:8080