refactor: remove unused merge checker

## Summary

### Why?

SubmitQueue validation already crosses the Runway boundary, so the older in-process merge checker and its result entities have no callers and duplicate ownership of merge execution.

### What?

Remove the mergechecker contract and implementations, delete the orphaned MergeResult and PushResult entities, and stop generating mergechecker mocks.

## Test Plan

- ✅ `make gazelle`
- ✅ `make test`

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: behinddwalls

Makefile

@@ -579,7 +579,7 @@ local-stovepipe-stop: ## Stop the Stovepipe service
 
 mocks: ## Generate mock files using mockgen
 	@echo "Generating mocks..."
-	@$(BAZEL) run @rules_go//go -- generate ./submitqueue/extension/storage/... ./submitqueue/extension/buildrunner/... ./submitqueue/extension/changeprovider/... ./platform/extension/counter/... ./platform/extension/consumergate/... ./platform/extension/hook/... ./platform/extension/messagequeue/... ./submitqueue/extension/queueconfig/... ./submitqueue/extension/mergechecker/... ./submitqueue/extension/conflict/... ./submitqueue/extension/speculation/... ./submitqueue/extension/validator/... ./platform/consumer/... ./stovepipe/core/requestlog/... ./stovepipe/extension/storage/... ./stovepipe/extension/sourcecontrol/...
+	@$(BAZEL) run @rules_go//go -- generate ./submitqueue/extension/storage/... ./submitqueue/extension/buildrunner/... ./submitqueue/extension/changeprovider/... ./platform/extension/counter/... ./platform/extension/consumergate/... ./platform/extension/hook/... ./platform/extension/messagequeue/... ./submitqueue/extension/queueconfig/... ./submitqueue/extension/conflict/... ./submitqueue/extension/speculation/... ./submitqueue/extension/validator/... ./platform/consumer/... ./stovepipe/core/requestlog/... ./stovepipe/extension/storage/... ./stovepipe/extension/sourcecontrol/...
 	@echo "Mocks generated successfully!"
 
 proto: ## Generate protobuf files from .proto definitions

submitqueue/entity/BUILD.bazel

@@ -13,9 +13,7 @@ go_library(
         "conflict.go",
         "land.go",
         "list.go",
-        "merge_result.go",
         "path_build.go",
-        "push_result.go",
         "queue_batch_state.go",
         "queue_config.go",
         "request.go",