opgeth-decoupling: final cutover (flip go.mod to upstream go-ethereum)

Part of #20257. The last step after all other sub-issues land.

Goal

Remove the op-geth replace directive from go.mod; clean up any remaining shims or transitional code.

Scope

• Remove from go.mod: replace github.com/ethereum/go-ethereum => github.com/ethereum-optimism/op-geth vX.Y.Z.
• Update the direct github.com/ethereum/go-ethereum dependency to the target upstream version.
• Run go mod tidy; resolve any residual import issues.
• Remove ALL op-core differential tests (types, fees, params, sync-tester PayloadID) and the §2 transaction-helper scaffolding — no longer meaningful without op-geth as a comparison point.
• op-core/params.GethChainConfig() sheds the OP fork fields and the Optimism struct from the produced go-ethereum ChainConfig (they only exist on the fork).
• op-core/interop/messages switches params.InteropCrossL2InboxAddress (fork const) to op-core/predeploys.
• Flip the log-layer alias to the owned interface (#21745 phase 2).
• Update CI / build configs that referenced op-geth; delete the CI ratchet job + baseline (#21748).
• Verify go build ./... and go test ./....
• Update CLAUDE.md / AGENTS.md pointer text (after this lands, the plan doc is historical reference).

Dependencies

Blocked on all other sub-issues, notably:

• #20263, #20264, #20265 — call-site swaps, OP-aware sources client, test migration.
• #20415 + #21196 — op-e2e/actions track (hard prerequisite). op-e2e/actions embeds op-geth in-process; flipping go.mod before it lands would silently re-resolve those imports to upstream go-ethereum with incorrect OP EVM behaviour.
• #21743 — Rust script engine for op-chain-ops/script + op-deployer. Fork-only EVM hooks; no module-split escape hatch (any in-repo module still on op-geth keeps the fork maintained, defeating the epic's purpose). Spikes 1–3 complete; production landing + bake remain.
• #21744 — op-reth-only L2 EL in op-e2e system tests + sysgo (otherwise the flip silently runs upstream geth as the L2 EL with wrong OP semantics — no compile error to warn).
• #21745 — foundations: own the log layer + re-home the RPC recorder (fork log/rpc extensions break the whole tree at flip time).
• #21746 (check-*), #21747 (op-wheel/cheat), #21281 (genesis), #21282 (op-simulate/op-run-block deletion), #21525 (sync-tester PayloadID).

The CI ratchet (#21748) is the readiness gauge: this issue is executable exactly when the ratchet baseline is empty; the flip PR deletes the job.

🤖 Generated by Claude Code

First check that the listed prerequisite issues have landed and that the CI ratchet baseline is empty. Then inspect go.mod and the named op-core, CI/build, log-layer, and documentation areas, running go mod tidy as the dependency changes proceed. Done means no op-geth shims or differential tests remain, and go build ./... plus go test ./... pass.