test(caprock): assert KernelSessionStruct accepts the session.create response

Guards the caprock RPC boundary against drifting from the daemon's
`session.create` / `session.list` response shape. The struct is a strict
`object()`, so an undeclared field makes `assert(result, KernelSessionStruct)`
in `createKernelSession` throw — which silently breaks the SessionStart and
PreToolUse hooks (native permission prompts, no TUI routing).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: grypez

packages/caprock/src/structs.test.ts

@@ -0,0 +1,36 @@
+import { is } from '@metamask/superstruct';
+import { describe, expect, it } from 'vitest';
+
+import { KernelSessionStruct } from './structs.ts';
+
+describe('KernelSessionStruct', () => {
+  // These fixtures mirror the object the daemon returns from `session.create`
+  // (and each entry of `session.list`) in
+  // packages/kernel-node-runtime/src/daemon/rpc-socket-server.ts. The struct is
+  // a strict `object()`, so it rejects any field it does not declare — if the
+  // daemon adds a field to that response, extend both the struct and this
+  // fixture. A prior omission of `lastActiveAt` made every `session.create`
+  // response fail `assert(result, KernelSessionStruct)` in
+  // `createKernelSession`, which broke the SessionStart and PreToolUse hooks.
+  it.each([
+    {
+      shape: 'full response',
+      response: {
+        sessionId: 'alice',
+        ocapUrl: 'ocap://kernel/session/alice',
+        cwd: '/home/user/project',
+        startedAt: '2026-07-01T00:00:00.000Z',
+        lastActiveAt: '2026-07-01T00:05:00.000Z',
+      },
+    },
+    {
+      shape: 'minimal response (optional fields absent)',
+      response: {
+        sessionId: 'bob',
+        ocapUrl: 'ocap://kernel/session/bob',
+      },
+    },
+  ])('accepts the daemon session.create $shape', ({ response }) => {
+    expect(is(response, KernelSessionStruct)).toBe(true);
+  });
+});