Cross-SDK DagSerialization compatibility CI for the Go SDK

### Background ADR 0003 explicitly promises: > The Go SDK gains a `serde` package that performs this encoding from > `bundlev1.Bundle` / `bundlev1.Task`, validated against > `validation/serialization/test_dags.yaml` (the same fixture set the > Java SDK uses), so the Go and Java outputs are byte-identical for > shared inputs. The Go `serde` package exists (`go-sdk/pkg/execution/serde.go`, `serde_test.go`) but the cross-SDK parity hook does **not** - there is no prek/CI step that runs the Go serializer against the shared `test_dags.yaml` and diffs the output against Python's `SerializedDAG.serialize_dag` like the Java SDK does via `compare.py`. ### What needs to happen 1. Add a Go-side serializer driver that consumes `validation/serialization/test_dags.yaml` and emits a JSON file per Dag fixture (parallel to the Java `SerializationCompatibilityTest`). 2. Wire it into `scripts/ci/prek/` so the existing `compare.py` runs against `serialized_go.json` alongside `serialized_java.json`. 3. Fail the check on any field-level divergence. ### Acceptance criteria - A regression in `SerializedDAG.serialize_dag` shape that Go doesn't follow fails CI before merge. - The Go and Java SDKs share the fixture file (no Go-only copy). ### Context - ADR: `go-sdk/adr/0003-coordinator-protocol-msgpack-ipc.md`. - Java parallel: #65959 introduced `check_java_serialization_compatibility.py`; we extend the same pattern. - Coordinator-distribution prek hooks tracking issue: #66515. --- Drafted-by: Claude Code (Opus 4.7) (no human review before posting)