feat: implement skills system for AI agents (#16865)

## Summary
This PR introduces a Skills system for AI agents, inspired by the [Agent
Skills specification](https://agentskills.io/specification).

## Changes

### Backend
- **SkillEntity**: New database entity with migration for storing skills
- **V2 Sync Mechanism**: Implemented FlatSkill, builders, validators,
and action handlers following the v2 flat entity pattern
- **Standard Skills**: Pre-defined skills (workflow-building,
data-manipulation, dashboard-building, metadata-building, research,
code-interpreter, xlsx, pdf, docx, pptx)
- **GraphQL API**: CRUD operations for skills with proper guards and
permissions
- **Workspace Cache**: Integrated skills into the workspace cache system

### Frontend  
- **Skills Table**: Searchable table in AI settings showing all skills
- **Skill Form**: Create/edit page with Label (primary), Description,
and Content (markdown editor)
- **API Name**: Following existing patterns, name is derived from label
with advanced settings toggle for custom API names
- **Standard vs Custom**: Standard skills are read-only, custom skills
can be edited/deleted

## Key Design Decisions
- Skills are stored in the database (Salesforce-like approach) rather
than files
- Name is derived from Label by default (isLabelSyncedWithName pattern)
- Skills reference functions/files via @ mentions in markdown content
rather than explicit relations
- Standard skills are synced from code, custom skills are created via UI

## Screenshots
Skills table and form UI follow existing settings patterns.

## Testing
- [x] Lint passes
- [x] Typecheck passes
- [ ] CI tests

packages/twenty-front/src/modules/app/components/SettingsRoutes.tsx

@@ -171,6 +171,12 @@ const SettingsAgentTurnDetail = lazy(() =>
   })),
 );
 
+const SettingsSkillForm = lazy(() =>
+  import('~/pages/settings/ai/SettingsSkillForm').then((module) => ({
+    default: module.SettingsSkillForm,
+  })),
+);
+
 const SettingsWorkspaceMembers = lazy(() =>
   import('~/pages/settings/members/SettingsWorkspaceMembers').then(
     (module) => ({
@@ -440,6 +446,14 @@ export const SettingsRoutes = ({ isAdminPageEnabled }: SettingsRoutesProps) => (
           path={SettingsPath.AIAgentTurnDetail}
           element={<SettingsAgentTurnDetail />}
         />
+        <Route
+          path={SettingsPath.AINewSkill}
+          element={<SettingsSkillForm mode="create" />}
+        />
+        <Route
+          path={SettingsPath.AISkillDetail}
+          element={<SettingsSkillForm mode="edit" />}
+        />
         <Route path={SettingsPath.Billing} element={<SettingsBilling />} />
         <Route path={SettingsPath.Domain} element={<SettingsDomain />} />
         <Route