Add node-facts.sh script with JSON node facts emission

Implements node-facts.sh that emits JSON documents for monitoring and inventory:
- Reads COMPOSE_FILE from .env and iterates only those compose files
- Fails fast with JSON error when .env missing or COMPOSE_FILE empty (exit 2)
- Uses --max-time for all curl calls (5s general, 10s for peers)
- Bounded parallel worker count for node processing
- Supports avalanchego, op-node/kona, beacon REST, geth-family, besu, reth/op-reth
- Includes fixture-only test-node-facts.sh with schema validation
- Adds README entry for the new script

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

node-facts-test/ethereum-mainnet-geth-pruned.yml

@@ -0,0 +1,11 @@
+version: "3"
+services:
+  client:
+    image: ethereum/client-go:v1.13.0
+    labels:
+      - "traefik.http.routers.ethereum-mainnet-geth-pruned.rule=PathPrefix(`/ethereum-mainnet-geth-pruned`)"
+      - "traefik.http.middlewares.ethereum-mainnet-geth-pruned.stripprefix.prefixes=/ethereum-mainnet-geth-pruned"
+    x-upstreams:
+      network: ethereum
+      chain: ethereum-mainnet
+chain: ethereum-mainnet