feat: Auto-register ai-skills for extensions whenever applicable (#1840)

* feat: Auto-register ai-skills for extensions whenever applicable

* fix: failing test

* fix: address copilot review comments – path traversal guard and use short_name in title

* fix: address remaining copilot review comments – is_file guard, skills type-validation, and exact extension ownership check on fallback rmtree

* fix: address copilot round-3 comments – align skill naming with presets.py convention, safe rmdir on fail, require SKILL.md for fallback rmtree, normalize skill_count in CLI

* fix: is_dir() guard in fast-path rmtree and fix ghost-skill assertion naming

* fix: path-traversal guard on skill_name in both rmtree paths of _unregister_extension_skills

* fix: add SKILL.md ownership check to fast-path rmtree and alias shadowed _get_skills_dir import