docs: document Reth port and log level variables in env files

[erhnysr]

Summary

Adds commented-out documentation for Reth-specific port and log level variables that are supported by reth-entrypoint but were completely absent from the env files.

Problem

Operators who want to customize Reth ports (e.g. running multiple nodes, resolving port conflicts) or change log verbosity had no way to discover these variables without reading the entrypoint source code directly.

Changes

Added documented, commented-out entries to both .env.mainnet and .env.sepolia:

Variable	Default	Description
RPC_PORT	8545	HTTP JSON-RPC port
WS_PORT	8546	WebSocket JSON-RPC port
AUTHRPC_PORT	8551	Engine API (authenticated RPC) port
METRICS_PORT	6060	Prometheus metrics port
DISCOVERY_PORT	30303	P2P discovery port
V5_DISCOVERY_PORT	9200	discv5 discovery port
P2P_PORT	30303	P2P TCP port
LOG_LEVEL	info	Reth log verbosity (error/warn/info/debug/trace)

Verification

All variables verified against reth/reth-entrypoint source.

[cb-heimdall]

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