Adding / as an allowed directory on linux does not work

On linux, if you add `/` as an allowed directory, it is not respected. E.g in mcphost
```
  *  System Loaded 5 tools from MCP servers
  >  User dump the contents of /etc/hostname
  [  filesystem__read_file /etc/hostname
  ]  Error Error: access denied - path outside allowed directories: /etc/hostname  
```
I will make a PR.
