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/fluksio/flow/store.py

@@ -94,6 +94,9 @@ def _same_content(left: FlowDef, right: FlowDef) -> bool:
 class FlowStore:
     """Reads and writes flows, committing every change."""
 
+    #: One warning per process when there is no git to commit with.
+    _warned_no_git = False
+
     def __init__(self, root: Path) -> None:
         self.root = root
         self.root.mkdir(parents=True, exist_ok=True)
@@ -109,12 +112,27 @@ class FlowStore:
     # -------------------------------------------------------------------------
 
     def _git(self, *args: str) -> subprocess.CompletedProcess[str]:
-        return subprocess.run(
-            ["git", "-C", str(self.root), *args],
-            capture_output=True,
-            text=True,
-            check=False,
-        )
+        try:
+            return subprocess.run(
+                ["git", "-C", str(self.root), *args],
+                capture_output=True,
+                text=True,
+                check=False,
+            )
+        except FileNotFoundError:
+            # No git on this machine — a `pip install` on a locked-down host is
+            # where this happens. The flows are files either way, which is what
+            # the store is; what is lost is their history, so say it once and
+            # carry on rather than refusing to start.
+            if not FlowStore._warned_no_git:
+                FlowStore._warned_no_git = True
+                logger.warning(
+                    "git is not installed, so flow changes are not versioned. "
+                    "Install it to get a commit per save."
+                )
+            return subprocess.CompletedProcess(
+                args=list(args), returncode=1, stdout="", stderr="git is not installed"
+            )
 
     def _commit(self, message: str, allow_empty: bool = False) -> None:
         self._git("add", "-A")
@@ -129,8 +147,12 @@ class FlowStore:
             "-m",
             message,
         )
-        if result.returncode != 0 and "nothing to commit" not in result.stdout:
-            logger.warning("Could not commit flow change: %s", result.stdout.strip())
+        if result.returncode == 0 or "nothing to commit" in result.stdout:
+            return
+        if result.stderr == "git is not installed":
+            # Already said once, in `_git`. Repeating it per save is noise.
+            return
+        logger.warning("Could not commit flow change: %s", result.stdout.strip())
 
     # -------------------------------------------------------------------------
     # Paths