Shell command checker can treat some plain heredocs as simple

A heredoc is a way of passing several lines of text to a shell command. Before Claude Code runs a shell command, it reads the command to understand it. Previously any heredoc not wrapped in quotes counted as too complex to check.

A new option changes this. When it is on, a heredoc whose only special parts are simple variables, such as $NAME, can be treated as simple. Heredocs inside a command substitution, which is a command run inside another command, are refused in this mode.