Function renderEnvExample

Renders a deterministic .env.example-style file scoped to the current configuration: every unique variable the active contracts require (one live entry per key, alphabetical; a key declared by more than one active contract renders once live and the rest commented-out with a pointer), followed by variables unique to disabled contracts (commented-out, for visibility -- a key already required by an active contract is not unique and never repeated here). reconciliationHeader, when non-empty, is spliced in right after the banner (see computeReconciliation).