fix: start neovim inside tmux by default

Author: devatnull

backend/Dockerfile.sandbox

@@ -42,8 +42,8 @@ WORKDIR /home/learner/practice
 # Switch to non-root user
 USER learner
 
-# Default command
-CMD ["tmux", "new-session", "-s", "learn"]
+# Default command - start tmux with neovim in the first window
+CMD ["tmux", "new-session", "-s", "learn", "nvim", "welcome.txt"]
 
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 # Security: This container runs with these restrictions (applied at runtime):