feat(stovepipe): define the repair executor extension

Summary:
Add RepairExecutor.Submit accepting an existing Build. Define queue-scoped
factory resolution and submission acceptance/error semantics, with generated
mocks and Bazel targets.

Extract this independently landable contract from the broader design in #893
so provider adapters can implement it before the controller is enabled.

Revert Plan:
Revert this commit.

Author: mnoah1

Makefile

@@ -629,6 +629,7 @@ GO_GENERATE_PACKAGES := \
 	./stovepipe/core/requestlog/... \
 	./stovepipe/extension/buildissue/... \
 	./stovepipe/extension/buildrunner/... \
+	./stovepipe/extension/fixer/... \
 	./stovepipe/extension/projectstatus/... \
 	./stovepipe/extension/queueconfig/... \
 	./stovepipe/extension/sourcecontrol/... \

stovepipe/extension/fixer/BUILD.bazel

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

stovepipe/extension/fixer/README.md

@@ -0,0 +1,9 @@
+# Repair execution
+
+[`RepairExecutor`](fixer.go) submits repair work for an existing [`Build`](../../entity/build.go). The executor selects which issues to repair and can invoke an in-process implementation or wrap an external provider.
+
+`Submit` returns promptly with an opaque provider attempt ID, or an empty ID if no repair is needed. Acceptance does not establish recovery.
+
+Each call may start an attempt. The caller owns first-failure selection and duplicate suppression. An error, including a timeout or cancellation, can occur after the provider accepted the work, so it must not cause an automatic resubmission. Implementations return plain dependency errors and must support concurrent calls.
+
+`Factory.For` resolves an executor bound to `Config.QueueName`. Generated mocks are available in [`mock`](mock/fixer_mock.go).

stovepipe/extension/fixer/fixer.go

@@ -0,0 +1,45 @@
+// Copyright (c) 2026 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package fixer defines repair submission for a build.
+package fixer
+
+//go:generate mockgen -source=fixer.go -destination=mock/fixer_mock.go -package=mock
+
+import (
+	"context"
+
+	"github.com/uber/submitqueue/stovepipe/entity"
+)
+
+// RepairExecutor submits repair work for a build within a configured queue.
+// Implementations must be concurrency-safe and return promptly after submission.
+type RepairExecutor interface {
+	// Submit returns an opaque provider attempt ID on acceptance, or empty if no repair
+	// is needed. Acceptance does not establish recovery.
+	// Repeated calls may start additional attempts. A plain error can follow acceptance,
+	// including on context cancellation; callers must not automatically repeat Submit.
+	Submit(context.Context, entity.Build) (string, error)
+}
+
+// Config identifies the queue whose repair executor is requested.
+type Config struct {
+	// QueueName identifies the validation queue.
+	QueueName string
+}
+
+// Factory resolves queue-bound executors; routing belongs to service wiring.
+type Factory interface {
+	For(Config) (RepairExecutor, error)
+}

stovepipe/extension/fixer/mock/BUILD.bazel

@@ -0,0 +1,13 @@
+load("@rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["fixer_mock.go"],
+    importpath = "github.com/uber/submitqueue/stovepipe/extension/fixer/mock",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//stovepipe/entity:go_default_library",
+        "//stovepipe/extension/fixer:go_default_library",
+        "@org_uber_go_mock//gomock:go_default_library",
+    ],
+)