update claude.md: add git checkout restriction, simplify nix section

CLAUDE.md

@@ -12,6 +12,7 @@ This project builds presentation decks for Marp, supporting multiple courses.
 - Agent NEVER runs commands outside this folder
 - Agent NEVER runs build/deploy commands without explicit user request
 - Agent NEVER runs deploy commands (make deploy, scp, etc.) without explicit user permission
+- Agent NEVER runs `git checkout --` or `git restore` on files with uncommitted work. To undo specific changes, use targeted Edit operations instead.
 
 ## Critical File Protection
 
@@ -48,10 +49,7 @@ make deploy         # Deploy all (ASK FIRST!)
 ## Nix Flake Commands
 
 ```bash
-nix develop                           # Dev shell with all tools
-nix run .#qr -- "https://example.com" # Generate QR code
-nix run .#qr-slides -- 223015b        # QR for course
-nix run .#optimize-img -- <path>      # Optimize images
+nix develop                           # Dev shell with all tools (node 22, npm, make)
 ```
 
 ## Code Style Guidelines