PR #281 removed the shared openshell-cluster Docker network in favor of
the default bridge. This restores custom bridge networking but makes each
gateway use its own isolated network named openshell-cluster-{name},
matching the existing container/volume naming convention.
Changes:
- Add network_name() to constants.rs for per-gateway network naming
- Add ensure_network() with retry/backoff and force_remove_network()
parameterized by network name instead of a global constant
- Attach containers to their per-gateway network via network_mode
- Disconnect and remove the network during gateway destroy
- Wire ensure_network() into the deploy flow before ensure_volume()
- Update architecture docs to reflect per-gateway network isolation