test: use cli_runner fixture in flight-plan integration tests

[pofallon]

Context

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

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).