feat(plugin): add vcs backend registration api

packages/plugin/src/v2/effect/vcs.ts

@@ -0,0 +1,33 @@
+export * as Vcs from "./vcs.js"
+
+import { FileDiff } from "@opencode-ai/schema/file-diff"
+import { FileStatus, Mode } from "@opencode-ai/schema/vcs"
+import { Schema, type Effect, type Scope } from "effect"
+
+export class RegistrationError extends Schema.TaggedErrorClass<RegistrationError>()("Vcs.RegistrationError", {
+  type: Schema.String,
+  message: Schema.String,
+}) {}
+
+export interface AdapterScope {
+  readonly directory: string
+  readonly worktree: string
+  readonly store: string
+}
+
+export interface Adapter {
+  readonly status: () => Effect.Effect<readonly FileStatus[]>
+  readonly diff: (
+    mode: Mode,
+    options?: { readonly context?: number },
+  ) => Effect.Effect<readonly FileDiff.Info[]>
+}
+
+export interface Backend {
+  readonly type: string
+  readonly make: (scope: AdapterScope) => Adapter
+}
+
+export interface VcsDomain {
+  readonly register: (backend: Backend) => Effect.Effect<void, RegistrationError, Scope.Scope>
+}