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/core-modules/tool-provider/tools/load-skill.tool.ts

@@ -1,6 +1,6 @@
 import { z } from 'zod';
 
-import { type Skill } from 'src/engine/core-modules/skills/skills.service';
+import { type FlatSkill } from 'src/engine/metadata-modules/flat-skill/types/flat-skill.type';
 
 export const LOAD_SKILL_TOOL_NAME = 'load_skill';
 
@@ -25,7 +25,7 @@ export type LoadSkillResult = {
   message: string;
 };
 
-export type LoadSkillFunction = (names: string[]) => Skill[];
+export type LoadSkillFunction = (names: string[]) => Promise<FlatSkill[]>;
 
 export const createLoadSkillTool = (loadSkills: LoadSkillFunction) => ({
   description:
@@ -36,7 +36,7 @@ export const createLoadSkillTool = (loadSkills: LoadSkillFunction) => ({
   }): Promise<LoadSkillResult> => {
     const { skillNames } = parameters.input;
 
-    const skills = loadSkills(skillNames);
+    const skills = await loadSkills(skillNames);
 
     if (skills.length === 0) {
       return {