fix(skills): drive UI rename gating from server renamable flag

Expose authoritative renamable on skill list responses using the same
managed-root policy as renameSkill, drop the divergent UI path heuristic,
and remove an unused rejection-test fixture.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/vscode/src/opencodeConfig.ts

@@ -2885,6 +2885,8 @@ const isManagedSkillPath = (skillMdPath: string, workingDirectory?: string): boo
   return getManagedSkillRoots(workingDirectory).some((root) => isPathInside(skillDir, root));
 };
 
+export { isManagedSkillPath };
+
 export const renameSkill = (oldName: string, newName: string, workingDirectory?: string): void => {
   ensureSkillDirs();
   validateSkillName(newName);