feat(tui): initial impl of diff viewer (#28476)

packages/opencode/src/effect/runtime-flags.ts

@@ -15,6 +15,7 @@ export class Service extends ConfigService.Service<Service>()("@opencode/Runtime
   autoShare: bool("OPENCODE_AUTO_SHARE"),
   pure: bool("OPENCODE_PURE"),
   disableDefaultPlugins: bool("OPENCODE_DISABLE_DEFAULT_PLUGINS"),
+  diffViewer: bool("OPENCODE_DIFF_VIEWER"),
   disableChannelDb: bool("OPENCODE_DISABLE_CHANNEL_DB"),
   disableEmbeddedWebUi: bool("OPENCODE_DISABLE_EMBEDDED_WEB_UI"),
   disableExternalSkills: bool("OPENCODE_DISABLE_EXTERNAL_SKILLS"),