Tests that use age exe incorrectly report pass rather than skip on Windows

[davidmatson]

Multiple tests have logic such as:

        if (!File.Exists("/opt/homebrew/bin/age"))
            return; // Skip if age CLI not available

which incorrectly reports the tests as Passed rather than Skipped.

Consider using Xunit.SkippableFact to allow reporting these tests as skipped:
https://stackoverflow.com/a/35871507

Also, consider supporting Windows for these tests as well by looking for age.exe in a different location (perhaps C:\Program Files\age or C:\Program Files\bin).