fix(plugins): omitted metadata in a workspace.create.before result means unchanged

Same handoff as agent.create.before: a legacy hook that returned only
{ source, ... } dropped the caller's metadata and anything an earlier
hook added, and workspace provisioning then defaulted it to {}. The
omitted field is carried forward at each handoff; an explicit {} still
replaces.

packages/server/src/server/plugins/lifecycle/handlers.test.ts

@@ -125,6 +125,41 @@ test("a create hook that omits metadata passes on what an earlier hook added", a
   expect(seen).toEqual([{ original: true, "new.plugin": { ticket: 42 } }]);
 });
 
+test("a workspace create hook that omits metadata passes on what an earlier hook added", async () => {
+  const hooks = new PluginHookHandlers(() => {});
+  hooks.addHook("workspace.create.before", async ({ request }) => ({
+    ...request,
+    metadata: { ...request.metadata, "new.plugin": { ticket: 42 } },
+  }));
+  hooks.addHook("workspace.create.before", async ({ request }) => ({ source: request.source }));
+  const seen: unknown[] = [];
+  hooks.addHook("workspace.create.before", async ({ request }) => {
+    seen.push(request.metadata);
+  });
+  const result = await hooks.invoke(
+    "operation",
+    "workspace.create.before",
+    { source: { kind: "directory", path: "/project" }, metadata: { original: true } },
+    paseo,
+  );
+  expect(result).toMatchObject({ metadata: { original: true, "new.plugin": { ticket: 42 } } });
+  expect(seen).toEqual([{ original: true, "new.plugin": { ticket: 42 } }]);
+
+  const replacing = new PluginHookHandlers(() => {});
+  replacing.addHook("workspace.create.before", async ({ request }) => ({
+    ...request,
+    metadata: {},
+  }));
+  await expect(
+    replacing.invoke(
+      "operation",
+      "workspace.create.before",
+      { source: { kind: "directory", path: "/project" }, metadata: { original: true } },
+      paseo,
+    ),
+  ).resolves.toMatchObject({ metadata: {} });
+});
+
 test("a session-open hook that returns its own object without metadata leaves metadata unchanged", async () => {
   const hooks = new PluginHookHandlers(() => {});
   hooks.addHook("agent.open.before", async ({ request }) => ({

packages/server/src/server/plugins/lifecycle/index.ts

@@ -252,6 +252,12 @@ export async function validateBeforeResult<Name extends keyof PluginWireBeforeRe
     // Omitted means unchanged: the next hook sees what this one was handed.
     return { ...result, metadata: next.metadata ?? previous.metadata };
   }
+  if (name === "workspace.create.before") {
+    const previous = beforeSchemas["workspace.create.before"].parse(input);
+    const next = beforeSchemas["workspace.create.before"].parse(result);
+    // Omitted means unchanged; an explicit {} still replaces.
+    return { ...result, metadata: next.metadata ?? previous.metadata };
+  }
   if (name === "worktree.create.before") {
     const previous = beforeSchemas["worktree.create.before"].parse(input);
     const next = beforeSchemas["worktree.create.before"].parse(result);