feat(stovepipe): define the repair executor extension (#929)

## Why?
Add a RepairExecutor contract. For now, we have a Submit method, but
this contract could expand to include other functionality such as
inspecting the status of a submitted request. Internally, we'll use this
to submit a fix when detected.

## What?

- Add `fixer.RepairExecutor.Submit(context.Context, entity.OpenIssue)
(string, error)`.
- Add queue configuration, the factory interface, generated GoMock
mocks, and Bazel targets.
- Document that a nonempty attempt ID means submission was accepted.
Errors may follow acceptance, so callers must avoid automatic
resubmission.

Follow-up layers will add first-failure selection, submission/history
handling, and build-analysis trigger wiring.

## Test Plan

- Regenerate mocks and Bazel files with `make mocks gazelle`.
- Run `make fmt lint check-tidy check-gazelle`.
- Compile `//stovepipe/extension/fixer:go_default_library` and
`//stovepipe/extension/fixer/mock:go_default_library`.

## Revert Plan

Revert this change.

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 one [`OpenIssue`](../../entity/open_issue.go). The executor can invoke an in-process implementation or wrap an external provider.
+
+`Submit` returns promptly with a nonempty opaque provider attempt ID on acceptance, or an error. Acceptance does not establish recovery.
+
+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 an open issue.
+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 one open issue within a configured queue.
+// Implementations must be concurrency-safe and return promptly after submission.
+type RepairExecutor interface {
+	// Submit returns a nonempty opaque provider attempt ID on acceptance.
+	// 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.OpenIssue) (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",
+    ],
+)