Replace the onboarding AI chat feature flag with an environment variable (#23439)

Follow-up to #23199.

The AI-chat onboarding is an instance-level rollout decision, not a
per-workspace experiment, so `IS_ONBOARDING_AI_CHAT_ENABLED` becomes an
instance config variable (default `false`, editable from the admin
panel) exposed to the frontend through `ClientConfig`. The workspace
feature flag is deleted; leftover `featureFlag` rows are inert since the
column is plain text.

`IS_WORKSPACE_COMPANY_ENRICHMENT_ENABLED` is removed as redundant: the
PDL client already skips everything when no API key is set. Enrichment
now runs when the AI chat is on and `PEOPLE_DATA_LABS_API_KEY` is
configured.

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packages/twenty-server/src/engine/workspace-manager/dev-seeder/core/utils/seed-feature-flags.util.ts

@@ -50,11 +50,6 @@ export const seedFeatureFlags = async ({
         workspaceId: workspaceId,
         value: false,
       },
-      {
-        key: FeatureFlagKey.IS_ONBOARDING_AI_CHAT_ENABLED,
-        workspaceId: workspaceId,
-        value: false,
-      },
     ])
     .execute();
 };