feat(skill): add support for custom skill directories via config

[ian-pascoe]

Summary

• Adds skill.paths config option to specify additional directories for SKILL.md discovery
• Supports relative paths (resolved from project root), absolute paths, and ~ home expansion
• Gracefully handles non-existent paths without errors

Closes#8533

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found a potential duplicate:

Related PR:

• feat: add custom skills path configuration #6539: "feat: add custom skills path configuration" (feat: add custom skills path configuration #6539)

This PR appears to address the same feature—adding custom skills path configuration. Since PR #8543 is a newer implementation of this capability, there may be overlap in functionality or it could be a reimplementation of the earlier PR #6539.

You should verify whether PR #6539 was already merged, abandoned, or if #8543 is intended to supersede it with improved functionality.

[ian-pascoe]

@thdxr help me out and please review. This will help plugins ship skills alongside them

[ian-pascoe]

@rekram1-node Any chance you could look at this? Don't want to be a pain but I know plugin devs (including myself) would love this

[ian-pascoe]

Looks like this was completed. Thanks!