fix(server): leaving an agent no longer keeps the agent's old thread

Moving an agent task to Continue or Fresh session with no thread chosen sent
profileId: null and threadId: null, and the save coalesced the null back to
the agent's thread. Later runs then went into that conversation with its
model and modes. Leaving an agent with an explicit null thread now clears the
binding; an omitted threadId still keeps it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: jayleaton

apps/server/src/scheduledTasks/ScheduledTaskService.profile.test.ts

@@ -141,6 +141,21 @@ it.effect(
           overrideFields: ["runtimeMode"],
         });
         assert.equal(launches[1]?.runtimeMode, "approval-required");
+
+        // Leaving the agent for an unbound continuing thread drops the agent's thread.
+        const plain = yield* service.upsert({
+          ...input,
+          id: task.id,
+          profileId: null,
+          threadId: null,
+          threadMode: "continue",
+        });
+        assert.isUndefined(plain.task.profileId);
+        assert.isNull(plain.task.threadId);
+        yield* service.runNow({ id: task.id });
+        assert.equal(launches.length, 3);
+        assert.isUndefined(launches[2]?.profileSelection);
+        assert.equal(sends.length, 1);
       }).pipe(Effect.provide(ScheduledTaskService.layer.pipe(Layer.provide(dependencies))));
     }).pipe(Effect.provide(memoryDatabase)),
 );

apps/server/src/scheduledTasks/ScheduledTaskService.ts

@@ -1491,7 +1491,14 @@ export const layer = Layer.effect(
         profileId !== undefined && requestedThreadMode === "new_thread"
           ? "continue"
           : requestedThreadMode;
-      const keptThreadId = input.threadId ?? existingTask?.threadId ?? null;
+      // The agent owned its thread, so leaving it with an explicit null thread starts fresh.
+      const leftAgentUnbound =
+        input.profileId === null &&
+        input.threadId === null &&
+        existingTask?.profileId !== undefined;
+      const keptThreadId = leftAgentUnbound
+        ? null
+        : (input.threadId ?? existingTask?.threadId ?? null);
       // A bound thread belongs to its project, so moving the task leaves it for a new one.
       const movedOffThread =
         existingTask !== null &&