Allow to provide multiple projects in `--project-slug`

I want to restrict which projects my Agent has access to.

Unfortunately it is not possible to provide multiple projects in the `--project-slug` argument.

This makes starting of the MCP slow, as I need to spin up a connection for every project.

It would be great if project names could be comma separated i.e.

```bash
--project-slug project1,project2,project3,project4
```

This is similar to how it's possible to provide multiple skills separated by comma:

```bash
--skills=inspect,docs
```