c2f2111b extracted shared utilities from exo_bench.py into harness.py
but accidentally dropped the run_planning_phase function and
--danger-delete-downloads CLI argument in the process.

Restored run_planning_phase in harness.py (where its dependencies now
live) and re-added the --danger-delete-downloads argument to
add_common_instance_args. Re-wired the planning phase call in
exo_bench.py's main() before the benchmark loop.