fix: reconcile public-read runtime messaging with visibility enforcement #338

The help text for GITLAWB_PUBLIC_READ and the startup warning still state that per-repository private-read enforcement is unavailable. Repository- and path-scoped visibility enforcement now exists, so the runtime messaging gives operators incorrect guidance.\n\nUpdate crates/gitlawb-node/src/config.rs and crates/gitlawb-node/src/main.rs to describe the current behavior and clearly document any remaining limits of GITLAWB_PUBLIC_READ.\n\nAcceptance criteria:\n- configuration help no longer says read enforcement is unwired;\n- the startup warning reflects the actual visibility controls;\n- behavior and operator guidance are covered by tests or documented validation.\n\nFollow-up to #332.