fix(kernel-utils): allow passable default guards for async section interfaces

The async interface guard synthesized for a sheaf section must admit implicit
exo methods like __getDescription__ that @endo/exo adds to every discoverable
exo. Without passable default guards, those methods are rejected at dispatch
time, preventing sheafs from being sent across a CapTP connection.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: grypez

packages/kernel-utils/src/sheaf/sheafify.ts

@@ -224,7 +224,12 @@ export const sheafify = <
     const resolvedGuard = guard;
 
     const asyncMethodGuards = asyncifyMethodGuards(resolvedGuard);
-    const asyncGuard = M.interface(`${name}:section`, asyncMethodGuards);
+    const asyncGuard =
+      schema === undefined
+        ? M.interface(`${name}:section`, asyncMethodGuards)
+        : M.interface(`${name}:section`, asyncMethodGuards, {
+            defaultGuards: 'passable',
+          });
 
     let revoked = false;