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-client-sdk/src/metadata/generated/schema.graphql

@@ -1756,7 +1756,6 @@ enum FeatureFlagKey {
   IS_JUNCTION_RELATIONS_ENABLED
   IS_REST_METADATA_API_NEW_FORMAT_DIRECT
   IS_LOGIC_FUNCTION_PREBUILT_MODE_ENABLED
-  IS_ONBOARDING_AI_CHAT_ENABLED
   IS_SETTINGS_DISCOVERY_HERO_ENABLED
   IS_WORKFLOW_VERSION_IN_CORE_ENABLED
 }
@@ -1960,6 +1959,7 @@ type ClientConfig {
   isCloudflareIntegrationEnabled: Boolean!
   isClickHouseConfigured: Boolean!
   isWorkspaceSchemaDDLLocked: Boolean!
+  isOnboardingAiChatEnabled: Boolean!
   enterpriseInstanceType: String!
   maintenance: ClientConfigMaintenanceMode
 }