feat(mergechecker): adding mergechecker interface with Check

CLAUDE.md

@@ -202,3 +202,4 @@ See [doc/howto/TESTING.md](doc/howto/TESTING.md) for full testing guide.
 1. **Structured logging** — `zap.SugaredLogger` with `Debugw`/`Infow`/`Errorw(msg, key, val, ...)`. Never unstructured methods.
 2. **Interfaces for behavior, structs for data** — use interfaces for behavioral contracts (Consumer, Controller, Storage). Use structs for data containers, configs, and registries (TopicRegistry, SubscriptionConfig).
 3. **Value types over pointers** — prefer value types for structs, configs, and return values. Use `(T, bool)` to signal absence instead of `*T`. Pointers only when mutation or shared ownership is needed.
+4. **Errors for failures, not control flow** — reserve `error` returns for unexpected or infrastructure failures. Use result types (structs, bools) for expected outcomes like `(Result, error)` or `(T, bool)`. Avoid sentinel errors that represent non-failure states.

extension/mergechecker/BUILD.bazel

@@ -0,0 +1,9 @@
+load("@rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "mergechecker",
+    srcs = ["mergechecker.go"],
+    importpath = "github.com/uber/submitqueue/extension/mergechecker",
+    visibility = ["//visibility:public"],
+    deps = ["//entity"],
+)

extension/mergechecker/mergechecker.go

@@ -0,0 +1,21 @@
+package mergechecker
+
+import (
+	"context"
+
+	"github.com/uber/submitqueue/entity"
+)
+
+// MergeChecker predicts whether a request's changes can merge cleanly.
+type MergeChecker interface {
+	// Check is a fail-fast validation that optimistically assesses the
+	// mergeability of the request. A positive result does not guarantee
+	// that the changes will apply cleanly at merge finalization time.
+	Check(ctx context.Context, request entity.Request) (Result, error)
+}
+
+// Result holds the outcome of a merge check.
+type Result struct {
+	// Mergeable is true if the request's changes are expected to merge cleanly.
+	Mergeable bool
+}