skills: realpathSync per skill on every registry load #1456

Found during the v0.12.7 release review (tech lead). Deferred: fine at today's skill counts.

#1435 labels each skill's source by resolving its path with realpathSync once per skill on every registry load (skill/index.ts, after loadSkills): synchronous I/O inside an Effect that grows with the skill count. Revisit if workspaces ship hundreds of skills (cache by path, or resolve the roots once).