[FEATURE]: Support discovering Global Claude skills

[ZainRizvi]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

An extension to https://github.com/sst/opencode/issues/6177

Basically, it would be awesome if opencode could automatically support discovering globally configured claude code skills in addition to the project level ones it currently supports

Details in: https://github.com/sst/opencode/issues/6177#issuecomment-3697634780