arb/robinhood: add nitro pruned-pebble-path compose for mainnet and testnet

Orbit rollup on Ethereum/Sepolia with per-chain baseConfig assets (testnet
info-json from official config; mainnet genesis-json-file bootstrap).

Co-authored-by: Cursor <cursoragent@cursor.com>

arb/robinhood/mainnet/baseConfig.json

@@ -0,0 +1,8 @@
+{
+    "chain": {
+        "id": 4663
+    },
+    "init": {
+        "genesis-json-file": "/config/robinhood-genesis.json"
+    }
+}