fix(skills): validate installs before writing

Preserve bundled files and normalize fallback names across skill installation paths.

Validate complete batches before writing and reject existing targets to avoid partial installs.

Keep project metadata and make folder selection tolerant of casing and cancelled dialogs.

server/modules/providers/tests/skills.test.ts

@@ -568,6 +568,54 @@ test('providerSkillsService adds global skills for claude, codex, gemini, and cu
       'console.log("codex skill");\n',
     );
 
+    const fallbackNamedSkills = await providerSkillsService.addProviderSkills('codex', {
+      entries: [
+        {
+          fileName: 'fallback / skill.md',
+          content: '---\ndescription: Normalized fallback skill\n---\n\nFallback body.\n',
+        },
+      ],
+    });
+    const fallbackNamedSkill = fallbackNamedSkills[0];
+    assert.ok(fallbackNamedSkill);
+    assert.equal(fallbackNamedSkill.name, 'fallback-skill');
+    assert.equal(fallbackNamedSkill.command, '$fallback-skill');
+    assert.equal(
+      fallbackNamedSkill.sourcePath.endsWith(path.join('.agents', 'skills', 'fallback-skill', 'SKILL.md')),
+      true,
+    );
+
+    await assert.rejects(
+      providerSkillsService.addProviderSkills('codex', {
+        entries: [
+          {
+            directoryName: 'uploaded-codex-folder',
+            content: '---\nname: replacement\n---\n\nReplacement body.\n',
+          },
+        ],
+      }),
+      /already exists/i,
+    );
+    assert.match(await fs.readFile(createdCodexSkill.sourcePath, 'utf8'), /Codex body\./);
+
+    const pendingBatchSkillPath = path.join(tempRoot, '.agents', 'skills', 'pending-batch', 'SKILL.md');
+    await assert.rejects(
+      providerSkillsService.addProviderSkills('codex', {
+        entries: [
+          {
+            directoryName: 'pending-batch',
+            content: '---\nname: pending-batch\n---\n\nPending body.\n',
+          },
+          {
+            directoryName: 'pending-batch',
+            content: '---\nname: duplicate-batch\n---\n\nDuplicate body.\n',
+          },
+        ],
+      }),
+      /duplicate skill target/i,
+    );
+    await assert.rejects(fs.stat(pendingBatchSkillPath), { code: 'ENOENT' });
+
     const createdGeminiSkills = await providerSkillsService.addProviderSkills('gemini', {
       entries: [
         {