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-server/src/engine/workspace-cache/types/workspace-cache-key.type.ts

@@ -31,6 +31,7 @@ export const WORKSPACE_CACHE_KEYS_V2 = {
   flatRoleTargetMaps: 'flat-maps:role-target',
   ORMEntityMetadatas: 'orm:entity-metadatas',
   flatAgentMaps: 'flat-maps:agent',
+  flatSkillMaps: 'flat-maps:skill',
   flatRoleTargetByAgentIdMaps: 'flat-maps:flatRoleTargetByAgentId',
   flatPageLayoutMaps: 'flat-maps:page-layout',
   flatPageLayoutWidgetMaps: 'flat-maps:page-layout-widget',