feat: Implement Agent Skills MVP

Implements issue #155 - Agent Skills system following agentskills.io standard.

## Core Implementation

### Main Process
- Add skillsService with discovery, loading, enable/disable
- Add IPC handlers for all skills operations
- Integrate skills context into system prompt builder
- Auto-create ~/levante/skills/ directory on startup

### Types
- Add Skill, SkillMetadata, SkillsConfig types
- Add SkillValidationResult for validation

### Preload
- Add skillsApi with list, get, enable, disable, toggle, refresh
- Expose skills path getter

### Renderer
- Add skillsStore (Zustand) for state management
- Add SkillsPage with grid view of skills
- Add SkillCard component with toggle switch
- Add Skills item to sidebar navigation
- Add en/es translations for skills UI

### Dependencies
- Add gray-matter for YAML frontmatter parsing

## How It Works
1. Skills are folders in ~/levante/skills/ with a SKILL.md file
2. SKILL.md uses YAML frontmatter for metadata (name, description, etc)
3. Enabled skills are injected into the AI system prompt
4. Toggle skills on/off from the Skills page

## Skill Format (SKILL.md)
```yaml
---
name: my-skill
description: What this skill does
version: 1.0.0
author: Author Name
---

# Skill Instructions

Markdown content that gets injected into the AI context.
```

Closes #155

Author: Ubuntu

package.json

@@ -86,6 +86,7 @@
     "electron-store": "^11.0.2",
     "embla-carousel-react": "^8.6.0",
     "fix-path": "^5.0.0",
+    "gray-matter": "^4.0.3",
     "i18next": "^25.7.2",
     "iceberg-js": "^0.8.1",
     "input-otp": "^1.4.2",

src/main/ipc/skillsHandlers.ts

@@ -0,0 +1,170 @@
+/**
+ * Skills IPC Handlers
+ * 
+ * Handles IPC communication for the skills system.
+ */
+
+import { ipcMain } from 'electron';
+import { skillsService } from '../services/skills';
+import { getLogger } from '../services/logging';
+
+const logger = getLogger();
+
+export function registerSkillsHandlers(): void {
+  logger.core.debug('Registering skills IPC handlers');
+
+  // List all skills
+  ipcMain.handle('levante/skills/list', async () => {
+    try {
+      const skills = skillsService.getSkills();
+      return { success: true, data: skills };
+    } catch (error) {
+      logger.core.error('Failed to list skills', {
+        error: error instanceof Error ? error.message : error
+      });
+      return { 
+        success: false, 
+        error: error instanceof Error ? error.message : 'Failed to list skills' 
+      };
+    }
+  });
+
+  // Get a single skill
+  ipcMain.handle('levante/skills/get', async (_, id: string) => {
+    try {
+      const skill = skillsService.getSkill(id);
+      if (!skill) {
+        return { success: false, error: 'Skill not found' };
+      }
+      return { success: true, data: skill };
+    } catch (error) {
+      logger.core.error('Failed to get skill', {
+        id,
+        error: error instanceof Error ? error.message : error
+      });
+      return { 
+        success: false, 
+        error: error instanceof Error ? error.message : 'Failed to get skill' 
+      };
+    }
+  });
+
+  // Get enabled skills
+  ipcMain.handle('levante/skills/enabled', async () => {
+    try {
+      const skills = skillsService.getEnabledSkills();
+      return { success: true, data: skills };
+    } catch (error) {
+      logger.core.error('Failed to get enabled skills', {
+        error: error instanceof Error ? error.message : error
+      });
+      return { 
+        success: false, 
+        error: error instanceof Error ? error.message : 'Failed to get enabled skills' 
+      };
+    }
+  });
+
+  // Enable a skill
+  ipcMain.handle('levante/skills/enable', async (_, id: string) => {
+    try {
+      const result = await skillsService.enableSkill(id);
+      return { success: result };
+    } catch (error) {
+      logger.core.error('Failed to enable skill', {
+        id,
+        error: error instanceof Error ? error.message : error
+      });
+      return { 
+        success: false, 
+        error: error instanceof Error ? error.message : 'Failed to enable skill' 
+      };
+    }
+  });
+
+  // Disable a skill
+  ipcMain.handle('levante/skills/disable', async (_, id: string) => {
+    try {
+      const result = await skillsService.disableSkill(id);
+      return { success: result };
+    } catch (error) {
+      logger.core.error('Failed to disable skill', {
+        id,
+        error: error instanceof Error ? error.message : error
+      });
+      return { 
+        success: false, 
+        error: error instanceof Error ? error.message : 'Failed to disable skill' 
+      };
+    }
+  });
+
+  // Toggle a skill
+  ipcMain.handle('levante/skills/toggle', async (_, id: string) => {
+    try {
+      const result = await skillsService.toggleSkill(id);
+      return { success: result };
+    } catch (error) {
+      logger.core.error('Failed to toggle skill', {
+        id,
+        error: error instanceof Error ? error.message : error
+      });
+      return { 
+        success: false, 
+        error: error instanceof Error ? error.message : 'Failed to toggle skill' 
+      };
+    }
+  });
+
+  // Refresh skills (re-discover)
+  ipcMain.handle('levante/skills/refresh', async () => {
+    try {
+      const skills = await skillsService.refresh();
+      return { success: true, data: skills };
+    } catch (error) {
+      logger.core.error('Failed to refresh skills', {
+        error: error instanceof Error ? error.message : error
+      });
+      return { 
+        success: false, 
+        error: error instanceof Error ? error.message : 'Failed to refresh skills' 
+      };
+    }
+  });
+
+  // Get skills path
+  ipcMain.handle('levante/skills/path', async () => {
+    try {
+      const path = skillsService.getSkillsPath();
+      return { success: true, data: path };
+    } catch (error) {
+      return { 
+        success: false, 
+        error: error instanceof Error ? error.message : 'Failed to get skills path' 
+      };
+    }
+  });
+
+  // Validate a skill
+  ipcMain.handle('levante/skills/validate', async (_, id: string) => {
+    try {
+      const skill = skillsService.getSkill(id);
+      if (!skill) {
+        return { success: false, error: 'Skill not found' };
+      }
+      const result = skillsService.validateSkill(skill);
+      return { success: true, data: result };
+    } catch (error) {
+      logger.core.error('Failed to validate skill', {
+        id,
+        error: error instanceof Error ? error.message : error
+      });
+      return { 
+        success: false, 
+        error: error instanceof Error ? error.message : 'Failed to validate skill' 
+      };
+    }
+  });
+
+  logger.core.debug('Skills IPC handlers registered');
+}

src/main/lifecycle/initialization.ts

@@ -30,6 +30,8 @@ import { setupAttachmentHandlers } from "../ipc/attachmentHandlers";
 import { registerAnalyticsHandlers } from "../ipc/analyticsHandlers";
 import { setupWidgetHandlers } from "../ipc/widgetHandlers";
 import { setupAnnouncementHandlers } from "../ipc/announcementHandlers";
+import { registerSkillsHandlers } from "../ipc/skillsHandlers";
+import { skillsService } from "../services/skills";
 
 const logger = getLogger();
 
@@ -104,6 +106,17 @@ export async function initializeServices(): Promise<void> {
       error: error instanceof Error ? error.message : error,
     });
   }
+
+  // 6. Initialize skills service
+  try {
+    await skillsService.initialize();
+    logger.core.info("Skills service initialized successfully");
+  } catch (error) {
+    logger.core.error("Failed to initialize skills service", {
+      error: error instanceof Error ? error.message : error,
+    });
+    // Continue with degraded functionality - skills are optional
+  }
 }
 
 /**
@@ -131,6 +144,7 @@ export async function registerIPCHandlers(getMainWindow: () => BrowserWindow | n
   setupOAuthHandlers();
   setupWidgetHandlers();
   setupAnnouncementHandlers();
+  registerSkillsHandlers();
 
   logger.core.info("All IPC handlers registered successfully");
 }

src/main/services/ai/systemPromptBuilder.ts

@@ -204,6 +204,22 @@ Example response format:
 Click the button above to add it. Once configured, I'll be able to help you with GitHub operations."`;
   }
 
+  // Add enabled skills context
+  try {
+    const { skillsService } = await import('../skills');
+    const skillsContext = skillsService.getSkillsForPrompt();
+    if (skillsContext) {
+      systemPrompt += skillsContext;
+      logger.aiSdk.debug('Skills context added to system prompt', {
+        enabledSkillsCount: skillsService.getEnabledSkills().length
+      });
+    }
+  } catch (error) {
+    logger.aiSdk.debug('Failed to load skills context (skills may not be initialized)', {
+      error: error instanceof Error ? error.message : error
+    });
+  }
+
   // Debug log for final system prompt
   logger.aiSdk.debug('Final system prompt generated', {
     enabled: personalization?.enabled || false,

src/main/services/skills/index.ts

@@ -0,0 +1,5 @@
+/**
+ * Skills Service Exports
+ */
+
+export { skillsService } from './skillsService';