hive-c0re: validate forge env at startup; char-safe short revs

HIVE_FORGE_URL (set + parseable) and HIVE_ASSETS_DIR are checked once in
main before workers start. Short revs in container_view and pr_status
take 12 chars instead of byte-slicing.

hive-c0re/src/main.rs

@@ -219,6 +219,9 @@ async fn main() -> Result<()> {
             // succeeded.
             lifecycle::network_isolation_from_env()
                 .context("network isolation settings are required at startup")?;
+            // The forge integration is mandatory (the NixOS module sets both
+            // vars unconditionally), so check them before any worker starts.
+            forge::validate_env()?;
             cmd_serve(sc.env, sc.model_prices, sc.build_slots, db, &cli.socket).await
         }
     }