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>

README.md

@@ -29,6 +29,8 @@ fluksio enroll <code> --portal https://hub.fluksio.com   # watch it from the por
 
 Nothing else has to be running. `--data-dir` puts the installation somewhere else —
 worth it on a cluster, where `$HOME` is often a network filesystem SQLite cannot use.
+`git` is not required but is worth having: flows are files either way, and it is what
+turns each save into a commit.
 The dashboard is served by the portal, so a machine with no inbound route is reached
 without opening a port: it dials out.