Start without git, and say what that costs

A `pip install` on a locked-down host — the case the CLI exists for —
may have no git, and the store shelled out to it while building the flow
repository, so the engine refused to start at all. The store is files;
git is their history. Missing it is now one warning and no commits
rather than a stack trace, which is the difference between a machine
that runs your experiments and one that does not.

Found by installing the wheels into a bare python:3.12-slim and pairing
it with the portal: `fluksio enroll` took the code, `fluksio serve`
dialled out, and the hub was proxying requests through the tunnel.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/test_cli.py

@@ -34,3 +34,29 @@ def test_the_secret_key_is_kept_rather_than_regenerated(tmp_path: Path) -> None:
     first = load_or_create_secret_key(path)
     assert load_or_create_secret_key(path) == first
     assert path.stat().st_mode & 0o777 == 0o600
+
+
+def test_the_store_works_without_git(tmp_path: Path, monkeypatch) -> None:
+    """A pip install on a locked-down host may have no git.
+
+    Flows are files, and that is what the store is for; the history is the part
+    that needs git. Losing it must not be a refusal to start.
+    """
+    import subprocess as sp
+
+    from fluksio.flow.store import FlowStore
+
+    real_run = sp.run
+
+    def no_git(cmd, *args, **kwargs):
+        if cmd and cmd[0] == "git":
+            raise FileNotFoundError(2, "No such file or directory", "git")
+        return real_run(cmd, *args, **kwargs)
+
+    monkeypatch.setattr(sp, "run", no_git)
+    monkeypatch.setattr(FlowStore, "_warned_no_git", False)
+
+    store = FlowStore(tmp_path / "flows")
+    assert store.head() == ""
+    store.write_requirements("numpy\n")
+    assert store.read_requirements() == "numpy\n"