Replace Sepolia Erigon archive with pruned Reth

[KillariDev]

Summary

• replace the Sepolia Erigon archive deployment with pinned Reth and Lighthouse services
• retain block bodies, receipts, and transaction lookups from an operator-selected fixed block while pruning older state history
• generate and share the Engine API JWT through a persistent initialization volume
• document cutoff selection, safe migration, peer ports, RPC endpoints, retention tradeoffs, and rollback
• update augurScan to use the internal http://reth:8545 endpoint

Why

The Erigon deployment ran in archive mode and exceeded 1 TB. The replacement preserves eth_getLogs and receipt access from the configured start-of-month boundary without retaining archive state from genesis.

Validation

• parsed reth/compose.yaml and asserted required services, fixed retention boundaries, and peer port mappings
• checked every configured Reth flag against the pinned v2.2.0 source
• checked Lighthouse flags against the pinned v8.2.1 binary help
• bun run check:changed
• git diff --check
• documentation review: 96/100, no findings
• visual documentation review: 96/100, no findings
• final configuration review: 96/100, no findings
• branch current with origin/main before commit

Docker is not installed in the authoring environment, so container startup, synchronization, and historical RPC queries must be completed on the operator host before the old Erigon volume is retired. The old volume is not deleted or reused by this change.