fix(plugins): omitted metadata in a hook result means unchanged

An agent.create.before hook that returned its own { config, env } dropped
metadata that an earlier hook in the chain had added, because only the
caller's original metadata was restored at the end. The omitted field is
now carried forward at each hook handoff, as agent.open.before already
does.

The agent.open.before result type also required metadata, so an older
TypeScript plugin that returned its own object no longer compiled even
though the daemon accepted it. The result type now makes it optional.

packages/plugin/src/boundaries.test.ts

@@ -283,6 +283,10 @@ describe("lifecycle hook SDK types", () => {
         server.addHook("workspace.create.before", async ({ request }) => ({ ...request, title: "Changed" }));
         server.addHook("workspace.create.before", async () => {});
         server.addHook("agent.open.before", async () => {});
+        server.addHook("agent.open.before", async ({ request }) => {
+          const { metadata, ...withoutMetadata } = request;
+          return withoutMetadata;
+        });
         server.addHook("agent.turn.after", async ({ agent, outcome, timeline }, { paseo, signal }) => {
           const kind: "completed" | "failed" | "canceled" = outcome.kind;
           // @ts-expect-error Notification payloads have no creation request.

packages/plugin/src/server/lifecycle.ts

@@ -132,10 +132,17 @@ export interface PluginHookRequests {
   "workspace.create.before": Omit<WorkspaceCreateRequest, "type" | "requestId">;
 }
 
+/** What a hook may return: a field it leaves out stays as it was. */
+type PluginHookRequestResults = Omit<PluginHookRequests, "agent.open.before"> & {
+  "agent.open.before": Omit<PluginSessionOpenRequest, "metadata"> & {
+    metadata?: Readonly<Record<string, JsonValue>>;
+  };
+};
+
 export type PluginHooks = {
   [Name in keyof PluginHookRequests]: {
     request: PluginHookRequests[Name];
-    result: PluginHookRequests[Name];
+    result: PluginHookRequestResults[Name];
   };
 } & { [Name in keyof PluginHookEvents]: { payload: PluginHookEvents[Name] } } & {
   "worktree.create.after": { payload: { worktree: PluginHookWorktree } };