fix: strip 'skills/' prefix in _collect_bare_skill to prevent double nesting

[Jah-yee]

Summary

When a dependency has virtual_path='skills/javascript-typescript-jest', the bare skill collector in _collect_bare_skill was producing output paths like skills/skills/javascript-typescript-jest/SKILL.md instead of skills/javascript-typescript-jest/SKILL.md.

This happens because the slug (derived from virtual_path) already contains the skills/ prefix, and the code unconditionally prepends another skills/ to the output path.

Fix

Strip the skills/ prefix from the slug before prepending skills/ in the output path:

# Strip "skills/" prefix to avoid double nesting like "skills/skills/..."
if slug.startswith("skills/"):
    slug = slug[len("skills/"):]

Test added

test_virtual_path_with_skills_prefix_no_double_nesting in tests/unit/test_plugin_exporter.py covers this exact bug scenario.

Fixes #719.