Thread count should be equal to number of CPUs by default

Description

Currently, OpenROAD will boot with a default thread count of 1:

$ openroad
OpenROAD 26Q2-911-g731f8ff5a4
Features included (+) or not (-): +GPU +GUI +Python
This program is licensed under the BSD-3 license. See the LICENSE file for details.
Components of this program may be licensed under more restrictive licenses which must be honored.
% thread_count
1
%

Presumably, most users do not know about -threads n or forget to set it. (I did until recently)

Suggested Solution

I suggest that openroad -threads max should be the default behaviour.

Additional Context

This is in relation to my LibreLane PR: https://github.com/librelane/librelane/pull/937 - this is also okay, but I think it would make more sense for OpenROAD to use the maximum number of CPUs by default.

I'd be happy to implement this change if desired.