feat(agents): use inject completion behavior for remote agent backgrounding

Set completionBehavior: 'inject' on remote agent executions so their
full output is reinjected into the conversation when they complete in
the background, and the task auto-dismisses from the Ctrl+B panel.

packages/core/src/agents/remote-invocation.test.ts

@@ -850,6 +850,7 @@ describe('RemoteAgentInvocation', () => {
       const info = bgListener.mock.calls[0][0];
       expect(info.executionId).toBe(capturedExecutionId);
       expect(info.executionMethod).toBe('remote_agent');
+      expect(info.completionBehavior).toBe('inject');
       expect(info.injectionText).toContain(
         "Remote agent 'Test Agent' completed successfully",
       );

packages/core/src/agents/remote-invocation.ts

@@ -149,6 +149,7 @@ export class RemoteAgentInvocation extends BaseToolInvocation<
         return output ? `${header}\nOutput:\n${output}` : header;
       },
       agentLabel,
+      'inject',
     );
     // createExecution always produces a valid numeric ID
     const executionId = handle.pid!;