`.env.mainnet` and `.env.sepolia` ship a shared default `BASE_NODE_L2_ENGINE_AUTH_RAW` with no fail-fast guard

[erhnysr]

Both .env.mainnet and .env.sepolia commit the same hardcoded value for BASE_NODE_L2_ENGINE_AUTH_RAW — byte-identical across the two files and present since the env files were first added. This is a secret that is meant to be unique per deployment, but it ships as a working default. Current behaviour: consensus-entrypoint and execution-entrypoint check only that the variable is non-empty, then use it as-is; because the committed default is non-empty, a deployment that never overrides it starts successfully and runs on the shared default with no warning or error. A safe default would ship a placeholder rather than a usable value, and the entrypoints would fail fast when the placeholder is still in place, so that running on an unconfigured secret is impossible rather than silent. Affected files: .env.mainnet, .env.sepolia, consensus-entrypoint, execution-entrypoint.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[erhnysr]

Still relevant — the shared hardcoded default with no fail-fast guard remains a real gap. Linked PR #1160 is ready for review.