test: use cli_runner fixture in flight-plan integration tests

## Context

`tests/integration/cli/test_flight_plan_commands.py` instantiates its own `CliRunner` in each test method:

```python
runner = CliRunner()
result = runner.invoke(cli, [...], catch_exceptions=False)
```

All other CLI unit tests use the shared `cli_runner` fixture from `conftest.py`. This inconsistency means the integration tests won't benefit from future enhancements to the fixture (e.g., `mix_stderr`, env vars, or isolation settings).

## Suggested fix

Update the integration test file to use the `cli_runner` fixture and `tmp_path` (or the `temp_dir` conftest fixture) for filesystem isolation, matching the pattern in the unit tests.

## Origin

Flagged during Python code review of branch `040-flight-plan-cli` (severity LOW).