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.test.js

@@ -224,7 +224,6 @@ describe('skills', () => {
     const managedDir = path.join(projectRoot, '.opencode', 'skills', 'managed-skill');
     const conflictDir = path.join(projectRoot, '.opencode', 'skills', 'taken-name');
     const mismatchDir = path.join(projectRoot, '.opencode', 'skills', 'folder-name');
-    const unmanagedDir = path.join(projectRoot, 'custom-skills', 'unmanaged-skill');
     const cacheStamp = `oc-rename-${Date.now()}`;
     const cacheDir = path.join(os.homedir(), '.cache', 'opencode', 'skills', cacheStamp, 'cache-skill');
 
@@ -274,21 +273,6 @@ describe('skills', () => {
         'utf8',
       );
 
-      await fsPromises.mkdir(unmanagedDir, { recursive: true });
-      await fsPromises.writeFile(
-        path.join(unmanagedDir, 'SKILL.md'),
-        [
-          '---',
-          'name: unmanaged-skill',
-          'description: Unmanaged',
-          '---',
-          '',
-          'Unmanaged body',
-          '',
-        ].join('\n'),
-        'utf8',
-      );
-
       await fsPromises.mkdir(cacheDir, { recursive: true });
       await fsPromises.writeFile(
         path.join(cacheDir, 'SKILL.md'),