Add `DEVIN_MODE` configuration supporting `enterprise` (default) and `organization_sessions` modes. Organization sessions mode uses `/v3/organizations/{org_id}/sessions` API with `ViewOrgSessions` permission, suitable for non-Enterprise organizations. Sums session ACUs consumed within configurable rolling window (1-365 days via `DEVIN_SESSION_WINDOW_DAYS`). Mode does not show quota, percentage, credit, or reset fields as these are unav