Add giant new-run to scaffold a config.toml + run dir ahead of training

Pulled forward from the not-yet-mergeable condor-gpu-train-rollout branch:
`new-run` resolves CLI hyperparameter overrides into a full config.toml and
run dir (reusing the existing default_out_dir_name collision-avoidance and a
newly factored-out router-override helper shared with `train`), so a run can
be prepared and reviewed before `giant train` actually kicks off. Also
brings README up to date with the model/CLI as it actually stands
(physical/embedding conditioning, WGAN/MoE-router modes, giant analyze,
W&B, setup-stage caching), which had drifted back to describing the
Phase-1 proof-of-concept.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_cli_new_run.py

@@ -0,0 +1,118 @@
+"""Tests for `giant new-run` (config.toml + run-dir scaffolding)."""
+
+from __future__ import annotations
+
+import tomllib
+from pathlib import Path
+
+from typer.testing import CliRunner
+
+from giant.cli import app
+
+runner = CliRunner()
+
+
+def test_writes_config_with_overrides_applied(tmp_path: Path):
+    out_dir = tmp_path / "run1"
+    result = runner.invoke(
+        app,
+        [
+            "new-run",
+            "--out",
+            str(out_dir),
+            "--mode",
+            "ddpm",
+            "--hidden-dim",
+            "128",
+            "--n-blocks",
+            "4",
+            "--lr",
+            "0.0005",
+        ],
+    )
+    assert result.exit_code == 0, result.output
+
+    config_path = out_dir / "config.toml"
+    assert config_path.exists()
+    with open(config_path, "rb") as f:
+        cfg = tomllib.load(f)
+
+    assert cfg["train"]["mode"] == "ddpm"
+    assert cfg["train"]["lr"] == 0.0005
+    assert cfg["model"]["hidden_dim"] == 128
+    assert cfg["model"]["n_blocks"] == 4
+    # untouched defaults still present
+    assert cfg["train"]["epochs"] == 100
+    assert "router" in cfg["model"]
+
+    assert str(out_dir) in result.output
+    assert "<data.parquet>" in result.output
+    assert "giant train" in result.output
+
+
+def test_comment_and_provenance_recorded_in_meta(tmp_path: Path):
+    out_dir = tmp_path / "run2"
+    result = runner.invoke(
+        app,
+        ["new-run", "--out", str(out_dir), "--comment", "quick test"],
+    )
+    assert result.exit_code == 0, result.output
+
+    with open(out_dir / "config.toml", "rb") as f:
+        cfg = tomllib.load(f)
+
+    assert cfg["meta"]["comment"] == "quick test"
+    assert cfg["meta"]["created_by"] == "giant new-run"
+    assert "created_at" in cfg["meta"]
+    assert "git_hash" in cfg["meta"]
+
+
+def test_data_flag_fills_printed_next_step_commands(tmp_path: Path):
+    out_dir = tmp_path / "run3"
+    result = runner.invoke(
+        app,
+        ["new-run", "--out", str(out_dir), "--data", "/ceph/lbogner/train.parquet"],
+    )
+    assert result.exit_code == 0, result.output
+    assert "/ceph/lbogner/train.parquet" in result.output
+    assert "<data.parquet>" not in result.output
+
+
+def test_dry_run_writes_nothing(tmp_path: Path):
+    out_dir = tmp_path / "run4"
+    result = runner.invoke(
+        app,
+        ["new-run", "--out", str(out_dir), "--hidden-dim", "512", "--dry-run"],
+    )
+    assert result.exit_code == 0, result.output
+    assert "dry-run" in result.output
+    assert "hidden_dim = 512" in result.output
+    assert not out_dir.exists()
+
+
+def test_force_guard_refuses_to_clobber_existing_checkpoints(tmp_path: Path):
+    out_dir = tmp_path / "run5"
+    out_dir.mkdir()
+    (out_dir / "last.pt").touch()
+
+    result = runner.invoke(app, ["new-run", "--out", str(out_dir), "--mode", "ddpm"])
+    assert result.exit_code != 0
+    assert "already has last.pt" in result.output
+    assert not (out_dir / "config.toml").exists()
+
+    result = runner.invoke(
+        app, ["new-run", "--out", str(out_dir), "--mode", "ddpm", "--force"]
+    )
+    assert result.exit_code == 0, result.output
+    assert (out_dir / "config.toml").exists()
+
+
+def test_default_out_dir_used_when_out_omitted(tmp_path: Path, monkeypatch):
+    monkeypatch.chdir(tmp_path)
+    result = runner.invoke(app, ["new-run", "--hidden-dim", "64"])
+    assert result.exit_code == 0, result.output
+
+    checkpoints_dir = tmp_path / "checkpoints"
+    run_dirs = list(checkpoints_dir.iterdir()) if checkpoints_dir.exists() else []
+    assert len(run_dirs) == 1
+    assert (run_dirs[0] / "config.toml").exists()