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`