feat(platform): Add gitworkspace extension for pluggable git command execution

platform/extension/gitworkspace/BUILD.bazel

@@ -0,0 +1,8 @@
+load("@rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["gitworkspace.go"],
+    importpath = "github.com/uber/submitqueue/platform/extension/gitworkspace",
+    visibility = ["//visibility:public"],
+)

platform/extension/gitworkspace/gitworkspace.go

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+// 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 gitworkspace defines the pluggable interface for executing git
+// commands in a stateful workspace. The OSS implementation runs commands as
+// local subprocesses; alternative deployments wrap their backend's exec API
+// in the same contract.
+package gitworkspace
+
+//go:generate mockgen -source=gitworkspace.go -destination=mock/gitworkspace_mock.go -package=mock
+
+import "context"
+
+// Command is one command to execute in a git workspace.
+type Command struct {
+	// Alias identifies this command's Output in the response.
+	Alias string
+	// Bin is the executable name.
+	Bin string
+	// Args are the command-line arguments.
+	Args []string
+	// Stdin is optional standard input.
+	Stdin string
+}
+
+// Output is the result of one executed command.
+type Output struct {
+	// Alias matches the Command that produced this output.
+	Alias string
+	// ExitCode is the process exit code (0 = success, -1 = skipped).
+	ExitCode int32
+	// Stdout is the captured standard output.
+	Stdout string
+	// Stderr is the captured standard error.
+	Stderr string
+}
+
+// Workspace is a stateful git environment where command batches execute
+// sequentially. Commands within one Exec call run in order; a non-zero
+// exit code skips the remaining commands in that batch. State persists
+// across Exec calls on the same Workspace.
+type Workspace interface {
+	// Exec sends a batch of commands and returns their outputs.
+	Exec(commands []Command) ([]Output, error)
+	// Close releases the workspace and its resources.
+	Close() error
+}
+
+// Factory creates Workspace instances bound to a repository.
+type Factory interface {
+	// For returns a Workspace for the given repository.
+	For(ctx context.Context, repo string) (Workspace, error)
+}

platform/extension/gitworkspace/mock/BUILD.bazel

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+load("@rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["gitworkspace_mock.go"],
+    importpath = "github.com/uber/submitqueue/platform/extension/gitworkspace/mock",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//platform/extension/gitworkspace:go_default_library",
+        "@org_uber_go_mock//gomock:go_default_library",
+    ],
+)