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-entity/constant/all-flat-entity-properties-to-compare-and-stringify.constant.ts

@@ -13,6 +13,7 @@ import { FLAT_ROLE_TARGET_EDITABLE_PROPERTIES } from 'src/engine/metadata-module
 import { FLAT_ROLE_EDITABLE_PROPERTIES } from 'src/engine/metadata-modules/flat-role/constants/flat-role-editable-properties.constant';
 import { FLAT_ROW_LEVEL_PERMISSION_PREDICATE_GROUP_EDITABLE_PROPERTIES } from 'src/engine/metadata-modules/flat-row-level-permission-predicate-group/constants/flat-row-level-permission-predicate-group-editable-properties.constant';
 import { FLAT_ROW_LEVEL_PERMISSION_PREDICATE_EDITABLE_PROPERTIES } from 'src/engine/metadata-modules/flat-row-level-permission-predicate/constants/flat-row-level-permission-predicate-editable-properties.constant';
+import { FLAT_SKILL_EDITABLE_PROPERTIES } from 'src/engine/metadata-modules/flat-skill/constants/flat-skill-editable-properties.constant';
 import { FLAT_VIEW_FIELD_EDITABLE_PROPERTIES } from 'src/engine/metadata-modules/flat-view-field/constants/flat-view-field-editable-properties.constant';
 import { FLAT_VIEW_FILTER_GROUP_EDITABLE_PROPERTIES } from 'src/engine/metadata-modules/flat-view-filter-group/constants/flat-view-filter-group-editable-properties.constant';
 import { FLAT_VIEW_FILTER_EDITABLE_PROPERTIES } from 'src/engine/metadata-modules/flat-view-filter/constants/flat-view-filter-editable-properties.constant';
@@ -135,6 +136,10 @@ export const ALL_FLAT_ENTITY_PROPERTIES_TO_COMPARE_AND_STRINGIFY = {
     ],
     propertiesToStringify: [],
   },
+  skill: {
+    propertiesToCompare: [...FLAT_SKILL_EDITABLE_PROPERTIES],
+    propertiesToStringify: [],
+  },
   rowLevelPermissionPredicate: {
     propertiesToCompare: [
       ...FLAT_ROW_LEVEL_PERMISSION_PREDICATE_EDITABLE_PROPERTIES,

packages/twenty-server/src/engine/metadata-modules/flat-entity/constant/all-metadata-many-to-one-relations.constant.ts

@@ -113,6 +113,7 @@ export const ALL_METADATA_RELATED_METADATA_BY_FOREIGN_KEY = {
     },
   },
   agent: {},
+  skill: {},
   pageLayout: {},
   pageLayoutWidget: {
     pageLayoutTabId: {

packages/twenty-server/src/engine/metadata-modules/flat-entity/constant/all-metadata-relations-properties.constant.ts

@@ -8,6 +8,10 @@ export const ALL_METADATA_RELATION_PROPERTIES = {
     workspace: true,
     application: true,
   },
+  skill: {
+    workspace: true,
+    application: true,
+  },
   fieldMetadata: {
     relationTargetFieldMetadata: true,
     relationTargetObjectMetadata: true,

packages/twenty-server/src/engine/metadata-modules/flat-entity/constant/all-metadata-required-metadata-for-validation.constant.ts

@@ -62,6 +62,7 @@ export const ALL_METADATA_REQUIRED_METADATA_FOR_VALIDATION = {
   agent: {
     role: true,
   },
+  skill: {},
   pageLayout: {},
   pageLayoutTab: {
     pageLayout: true,