Add a CLI "open" command

Add a CLI "open" command to address the problem of 'code ' opening that folder unless the folder name happens to be agent/chat/tunnel. I've found no way to open a folder with the same name as a sub-command from the CLI.

As an example 'code open agent' should launch code and open the folder named agent instead of trying to start/interact with the AI agent host as 'code agent' does.

Supporting 'code -- agent' would also be acceptable as the '--' pattern is a common convention to indicate the end of parameters or subcommands.