feat(skills): add edit modal, drag-and-drop upload, fix frontmatter stripping

- Add skill edit dialog (name, description, visibility, tags)
- Add drag-and-drop support to upload modal with visual feedback
- Fix frontmatter not stripped in skill detail view (CRLF normalization)
- Include visibility, tags, version in skill list/detail API responses
- Change default skill upload visibility from private to internal
- Add visibility column and version display to skills table

internal/http/skills.go

@@ -270,7 +270,7 @@ func (h *SkillsHandler) handleUpload(w http.ResponseWriter, r *http.Request) {
 		Slug:        slug,
 		Description: &desc,
 		OwnerID:     userID,
-		Visibility:  "private",
+		Visibility:  "internal",
 		Version:     version,
 		FilePath:    destDir,
 		FileSize:    size,