Fix CLI/tooling robustness gaps and dedupe the Conditioning enum

- run_create_manifest gains --force; it previously overwrote an
  existing manifest (including holdout.manifest, which
  check_holdout_overlap exists specifically to protect) with no
  warning or backup on a second run.
- _git_user_name only caught OSError, not subprocess.TimeoutExpired (a
  SubprocessError, not an OSError) — a slow/loaded shared portal
  machine could crash `dwarf bump-gen`/`bump-schema` instead of
  degrading to by=None as intended.
- `dwarf convert --jobs`/`make-root --jobs` now warn (never block) when
  the requested count exceeds ~1/4 of the machine's CPUs, matching the
  same shared-machine etiquette check added to giant train in the
  previous commit.
- The Conditioning enum was independently redefined in both
  giant/cli.py and scripts/dwarf.py; moved to a single
  giant.config.Conditioning both now import, removing the drift risk
  of a third conditioning mode being added to one but not the other.

Each fix has a regression test.

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

tests/test_dwarf.py

@@ -1,12 +1,35 @@
 from typer.testing import CliRunner
 
+from giant import cli as giant_cli
+from giant.config import Conditioning
 from giant.data import setup_cache
+from scripts import dwarf
 from scripts.dwarf import app
 from test_pipeline import _make_synthetic_steps
 
 runner = CliRunner()
 
 
+def test_conditioning_enum_shared_across_both_clis():
+    """giant.cli and scripts.dwarf must use the one giant.config.Conditioning
+    enum, not independently redefined copies that could silently drift apart
+    on valid --conditioning values."""
+    assert dwarf.Conditioning is Conditioning
+    assert giant_cli.Conditioning is Conditioning
+
+
+def test_warn_if_exceeds_shared_quota_warns_over_quarter_cpu(monkeypatch, capsys):
+    monkeypatch.setattr(dwarf.os, "cpu_count", lambda: 8)  # quota = 2
+    dwarf._warn_if_exceeds_shared_quota(3, "--jobs")
+    assert "warning: --jobs=3 exceeds" in capsys.readouterr().err
+
+
+def test_warn_if_exceeds_shared_quota_silent_within_quota(monkeypatch, capsys):
+    monkeypatch.setattr(dwarf.os, "cpu_count", lambda: 8)  # quota = 2
+    dwarf._warn_if_exceeds_shared_quota(2, "--jobs")
+    assert capsys.readouterr().err == ""
+
+
 def test_convert_rejects_jobs_below_one(tmp_path):
     root_file = tmp_path / "shard.root"
     root_file.touch()