[devops] .env.example not updated with testnet env vars — operators copying the file receive an opaque substitution error at startup

[obchain]

PR: #51 (feat/23-testnet-config)
File: .env.example
Refs #51

Problem

config/testnet.toml references two new env vars. The .env.example file contains only BNB_WS_URL and BNB_HTTP_URL (mainnet). An operator who:

1. Copies .env.example to .env
2. Runs charon --config config/testnet.toml listen

receives:

Error: env var `CHARON_BNB_TESTNET_WS_URL` is not set

with no hint in .env.example that this variable exists or what value to provide.

Impact

Friction for first-time testnet operators. The Grafana demo path described in the PR fails immediately after docker compose up with a cryptic error.

Fix

Add to .env.example:

# BSC testnet (Chapel) RPC endpoints — used with --config config/testnet.toml
# Public Chapel endpoint is rate-limited. Swap for QuickNode/Ankr Chapel for stable demo.
CHARON_BNB_TESTNET_WS_URL=wss://bsc-testnet.publicnode.com
CHARON_BNB_TESTNET_HTTP_URL=https://bsc-testnet.publicnode.com

Also document known reliable Chapel fallback endpoints (e.g. QuickNode Chapel, Ankr Chapel) given their notoriously high rate-limit rates on the public node.

[obchain]

Already fixed on feat/23-testnet-config (d0c5278, 3a562e4): .env.example now ships CHARON_BNB_TESTNET_WS_URL and CHARON_BNB_TESTNET_HTTP_URL with an inline note about Chapel rate limits and the CHARON_* namespace rationale (#245). Variable names match the substitutions in config/testnet.toml line 27-28.