status shows empty '(target: )' when no target is configured

## Summary

`agentcore status` renders an empty target label `(target: )` in the header when no deployment target is configured. Should either omit the label or show a default.

## Reproducer

```bash
agentcore create --name Test --defaults --skip-install --skip-git --skip-python-setup --json
cd Test
agentcore status
```

## Actual behavior

```
AgentCore Status (target: )
```

## Expected behavior

Either:
- `AgentCore Status` (omit target when empty)
- `AgentCore Status (target: default)` (show default label)

## Version

`agentcore 0.10.0-1777085255`

Run the provided agentcore create and agentcore status reproducer to confirm the empty target label. Trace the status command's header rendering, then make the output match one of the stated expected forms and verify agentcore status when no deployment target is configured.

索引モデルが issue の本文から書いたものです。