Add option to display reason for skipped sessions in the summary

[chirizxc]

How would this feature be useful?

In large projects, there can be multiple reasons why tests are skipped. For example, some tests might be skipped on Python 3.13 due to compatibility issues, while others could be skipped due to missing dependencies, platform-specific constraints, etc

Without a clear summary, developers often need to scroll through long test logs to find out why certain tests were skipped.

By adding a flag to display skip reasons in the final summary, developers can quickly understand why tests were skipped without digging through nox output.

Describe the solution you'd like

nox > Ran multiple sessions:
nox > * fail: failed
nox > * skip_3_13: skipped (Skip tests on python 3.13 due to compatibility issues)

Describe alternatives you've considered

No response

Anything else?

No response

[chirizxc]

I'm not sure about flag, but I think something like: --show-skip-reason-summary , --skip-summary-details

[chirizxc]

@DiddiLeija what do you think?