arb nitro minimal: self-prune via --init.prune=minimal in normal compose

Correct the minimal-node model: the minimal compose itself carries
--init.prune=minimal (prunes to genesis+head on start) instead of relying on a
separate .prune.yml — otherwise a minimal node was byte-identical to pruned in
normal operation and had no way to enforce minimal state. Removed the redundant
minimal .prune.yml files.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

arb/nitro/arbitrum-sepolia-nitro-minimal-pebble-hash.yml

@@ -62,6 +62,7 @@ services:
       - --http.corsdomain=*
       - --http.port=8545
       - --http.vhosts=*
+      - --init.prune=minimal
       - --metrics
       - --metrics-server.addr=0.0.0.0
       - --metrics-server.port=6070