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/metadata-modules/flat-skill/services/workspace-flat-skill-map-cache.service.ts

@@ -0,0 +1,44 @@
+import { Injectable } from '@nestjs/common';
+import { InjectRepository } from '@nestjs/typeorm';
+
+import { Repository } from 'typeorm';
+
+import { WorkspaceCacheProvider } from 'src/engine/workspace-cache/interfaces/workspace-cache-provider.service';
+
+import { type FlatSkillMaps } from 'src/engine/metadata-modules/flat-skill/types/flat-skill-maps.type';
+import { transformSkillEntityToFlatSkill } from 'src/engine/metadata-modules/flat-skill/utils/transform-skill-entity-to-flat-skill.util';
+import { createEmptyFlatEntityMaps } from 'src/engine/metadata-modules/flat-entity/constant/create-empty-flat-entity-maps.constant';
+import { SkillEntity } from 'src/engine/metadata-modules/skill/entities/skill.entity';
+import { WorkspaceCache } from 'src/engine/workspace-cache/decorators/workspace-cache.decorator';
+import { addFlatEntityToFlatEntityMapsThroughMutationOrThrow } from 'src/engine/workspace-manager/workspace-migration-v2/utils/add-flat-entity-to-flat-entity-maps-through-mutation-or-throw.util';
+
+@Injectable()
+@WorkspaceCache('flatSkillMaps')
+export class WorkspaceFlatSkillMapCacheService extends WorkspaceCacheProvider<FlatSkillMaps> {
+  constructor(
+    @InjectRepository(SkillEntity)
+    private readonly skillRepository: Repository<SkillEntity>,
+  ) {
+    super();
+  }
+
+  async computeForCache(workspaceId: string): Promise<FlatSkillMaps> {
+    const skills = await this.skillRepository.find({
+      where: { workspaceId },
+      withDeleted: true,
+    });
+
+    const flatSkillMaps = createEmptyFlatEntityMaps();
+
+    for (const skillEntity of skills) {
+      const flatSkill = transformSkillEntityToFlatSkill(skillEntity);
+
+      addFlatEntityToFlatEntityMapsThroughMutationOrThrow({
+        flatEntity: flatSkill,
+        flatEntityMapsToMutate: flatSkillMaps,
+      });
+    }
+
+    return flatSkillMaps;
+  }
+}