Allow using plugin Direct activation command as the query header (pt run/cmdpal)

### Description of the new feature / enhancement

It allows you to set a plugin's direct activation command as the query header (eg set the direct activation command to C:\ and then use that as the header instead of having to use C:\C:\... for PowerToys Run/CmdPal to start searching for files)

### Scenario when this would be used?

When you want to use the activation command as the start of the query

### Supporting information

_No response_

Start by tracing how PowerToys Run and CmdPal parse the query header and plugin direct activation command. Verify the current handling of activation commands such as `C:\`, then define tests for using that command as the header without requiring a duplicated prefix; done means both entry points start the intended file search.

Written by the indexing model from the issue text.