apply_stored_service_credentials

Export every stored service key into os.environ.

Services (e.g. web search via Tavily) have no base URL to reconcile, so this is a plain key copy onto the canonical env var name the underlying SDK reads. A stored key takes precedence over an existing plain env var, matching apply_stored_credentials; a DEEPAGENTS_CODE_-prefixed override is left authoritative because the app already treats it as the top-priority per-session credential.