fix(skill): follow Windows linked skill directories

[ferstar]

Summary

• Follow Windows reparse/irregular directory entries when scanning skill roots
• Keep symlinked skill directories and flat markdown skills discoverable under convention roots like ~/.agents/skills
• Add regression coverage for irregular directory entries resolving to skill folders

Fixes #4417

Tests

• go test ./internal/skill
• go test -run 'TestSkillRootsView|TestCapabilitiesIncludesDisabledSkills|TestNormalizeSkillPath' . from desktop/\n\nNote: local git hooks currently run npm at the repository root, where no package.json exists, so commit/push used --no-verify after the targeted Go tests passed.

[SivanCola]

Thanks @ferstar for this fix. I integrated the Windows linked skill directory behavior and regression tests into #4510 together with the overlapping Claude flat-Markdown warning work from #4441, because the two PRs conflict in either merge order.

Authorship note: @SivanCola is the maintainer integrator for #4510. @ferstar contributed as a repository contributor through this PR, and that contribution is preserved and acknowledged in #4510.

Closing this PR as superseded by #4510 so the repository has one conflict-free merge path.