fix: hand the jobs registry a session id, not the agent handle

JobSpec.owner is a SessionId, and the registry resolves it through
agents.get(owner) - so passing exec.agent made the DEFAULT index call fail
with 'session "[object Object]" has no live agent'. Since index defaults to
run_in_background, the action that exists to pre-index a workspace was the
one action that could not run. A handle without an id registers the job
unowned rather than throwing.

The test mounts a jobs service and pins the owner contract; no test had ever
supplied one, which is why this reached a real deployment.

index.js

@@ -1356,10 +1356,21 @@ export function apply(ctx, config) {
             )
 
           if (wantsBackground && jobs !== undefined) {
+            // `owner` is a SESSION ID, not the Agent handle.
+            //
+            // The jobs registry resolves an owner through the agent registry —
+            // `agents.get(session)` — and throws `session "[object Object]" has
+            // no live agent` for anything else, so passing `exec.agent` here made
+            // the default `index` call fail outright: it is the one path this
+            // plugin takes that a caller cannot see fail until the job service is
+            // present. The agent's own `id` IS the session id (the registry keys
+            // by it), and a handle without one is registered unowned rather than
+            // refused.
+            const owner = exec.agent?.id
             const jobId = jobs.start({
               kind: 'recall-index',
               label: `recall index ${workspace}`,
-              ...(exec.agent === undefined ? {} : { owner: exec.agent }),
+              ...(owner === undefined || owner === null || owner === '' ? {} : { owner }),
               run: () => {
                 const controller = new AbortController()
                 const done = (async () => {