* fix(deploy): fail fast when DEER_FLOW_HOME is not writable
`make docker-start` bind-mounts the host `backend/` directory into the gateway
container, whose process runs as root, so any state it creates under
`backend/.deer-flow` ends up owned by root on the host. A later `make up` runs
`scripts/deploy.sh` as the invoking user and only fails deep into the run with a
bare "Permission denied" while persisting generated secrets (deploy.sh:234).
Check writability right after `mkdir -p "$DEER_FLOW_HOME"` and, when it fails,
print the exact `chown` recovery command instead of letting the run get halfway
through. `make down` is excluded so stopping the stack is never blocked.
Fixes#6462
* test(deploy): cover runtime home writability preflight
* fix(deploy): diagnose persisted secret permissions and preserve teardown
---------
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>