analyze: default run directory to <repo>/analysis_runs, gitignored

giant analyze prep/submit previously defaulted the run directory to
next to the rollout parquet on /ceph. Default it instead to
<cwd>/analysis_runs/analysis_<id> so it lands inside the portal repo
checkout (/work) — gitignored, --run-dir still overrides it.
derive_run_dir/prep gained a default_base param; library callers that
don't pass one keep the old parquet-relative fallback.

tests/test_condor.py

@@ -89,6 +89,15 @@ def test_derive_run_dir_next_to_rollout():
     assert derive_run_dir(y, "/somewhere") == Path("/somewhere")
 
 
+def test_derive_run_dir_default_base():
+    y = {"output": "/data/roll.parquet", "prediction_id": "abcd1234ef", "dataset": "d"}
+    assert derive_run_dir(y, default_base="/work/lbogner/giant2/analysis_runs") == Path(
+        "/work/lbogner/giant2/analysis_runs/analysis_abcd1234"
+    )
+    # an explicit run_dir still wins over default_base
+    assert derive_run_dir(y, "/somewhere", default_base="/other") == Path("/somewhere")
+
+
 def test_prep_lays_out_run_dir(tmp_path: Path):
     yaml_path = _write_inputs(tmp_path)
     run_dir = _prep(yaml_path)