feat(skills): add provider skill management

Users need one settings surface to discover and install skills without manually navigating provider-specific directories.

Add provider-backed global skill installation for Claude, Codex, Gemini, and Cursor, while keeping OpenCode read-only because it reuses other providers' skill locations.

Add a responsive Skills settings tab with scoped discovery, search, refresh controls, markdown and folder uploads, upload feedback, and overflow-safe layouts.

Validate bundled skill files and paths before writing them, preserve scripts and assets, and cover provider discovery and installation behavior with tests.

server/modules/providers/services/skills.service.ts

@@ -1,5 +1,9 @@
 import { providerRegistry } from '@/modules/providers/provider.registry.js';
-import type { ProviderSkill, ProviderSkillListOptions } from '@/shared/types.js';
+import type {
+  ProviderSkill,
+  ProviderSkillCreateInput,
+  ProviderSkillListOptions,
+} from '@/shared/types.js';
 
 export const providerSkillsService = {
   /**
@@ -12,4 +16,15 @@ export const providerSkillsService = {
     const provider = providerRegistry.resolveProvider(providerName);
     return provider.skills.listSkills(options);
   },
+
+  /**
+   * Writes one or more global skills for one provider.
+   */
+  async addProviderSkills(
+    providerName: string,
+    input: ProviderSkillCreateInput,
+  ): Promise<ProviderSkill[]> {
+    const provider = providerRegistry.resolveProvider(providerName);
+    return provider.skills.addSkills(input);
+  },
 };