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/web/server/lib/opencode/skills.js

@@ -734,4 +734,5 @@ export {
   updateSkill,
   deleteSkill,
   renameSkill,
+  isManagedSkillPath,
 };