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-front/src/generated-admin/graphql.ts

@@ -326,7 +326,6 @@ export enum FeatureFlagKey {
   IS_JSON_FILTER_ENABLED = 'IS_JSON_FILTER_ENABLED',
   IS_JUNCTION_RELATIONS_ENABLED = 'IS_JUNCTION_RELATIONS_ENABLED',
   IS_LOGIC_FUNCTION_PREBUILT_MODE_ENABLED = 'IS_LOGIC_FUNCTION_PREBUILT_MODE_ENABLED',
-  IS_ONBOARDING_AI_CHAT_ENABLED = 'IS_ONBOARDING_AI_CHAT_ENABLED',
   IS_REST_METADATA_API_NEW_FORMAT_DIRECT = 'IS_REST_METADATA_API_NEW_FORMAT_DIRECT',
   IS_SETTINGS_DISCOVERY_HERO_ENABLED = 'IS_SETTINGS_DISCOVERY_HERO_ENABLED',
   IS_UNIQUE_INDEXES_ENABLED = 'IS_UNIQUE_INDEXES_ENABLED',