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/.env.example

@@ -86,10 +86,11 @@ FRONTEND_URL=http://localhost:3001
 # HTTP_TOOL_SAFE_MODE_ENABLED=true
 # ALLOW_REQUESTS_TO_TWENTY_ICONS=true
 
-# ———————— ENRICHMENT ————————
-# Enriches a new workspace with its own company data, used as AI chat context.
-# Disabled by default; requires both the flag and the API key.
-# IS_WORKSPACE_COMPANY_ENRICHMENT_ENABLED=true
+# ———————— ONBOARDING AI CHAT ————————
+# Opens an AI chat at the end of onboarding to help set up the workspace.
+# IS_ONBOARDING_AI_CHAT_ENABLED=true
+# Enriches the new workspace with its own company data, used as chat context.
+# Active as soon as the API key is set.
 # PEOPLE_DATA_LABS_API_KEY=
 
 # ———————— AI ————————