Add AI as a public feature flag in the Lab (#19277)

## Summary
- Adds `IS_AI_ENABLED` to `PUBLIC_FEATURE_FLAGS` so it appears in
**Settings > Lab** as a self-serve toggle for users
- Includes a cover image (`is-ai-enabled.png`) hosted on
`twenty.com/images/lab/`, following the existing convention for lab
feature flag images
- AI is now ready for public beta — users can enable it directly from
the Lab

## Test plan
- [ ] Verify the AI card appears in Settings > Lab with the cover image
banner
- [ ] Toggle AI on/off and confirm the feature flag is persisted
- [ ] Confirm AI features (agent chat, etc.) activate when the flag is
enabled


Made with [Cursor](https://cursor.com)

packages/twenty-server/src/engine/core-modules/feature-flag/constants/public-feature-flag.const.ts

@@ -12,6 +12,14 @@ export type PublicFeatureFlag = {
 };
 
 export const PUBLIC_FEATURE_FLAGS: PublicFeatureFlag[] = [
+  {
+    key: FeatureFlagKey.IS_AI_ENABLED,
+    metadata: {
+      label: 'AI',
+      description: 'Enable AI-powered features including the agent chat',
+      imagePath: 'https://twenty.com/images/lab/is-ai-enabled.png',
+    },
+  },
   {
     key: FeatureFlagKey.IS_JUNCTION_RELATIONS_ENABLED,
     metadata: {