fix(bootstrap): add systemd-networkd DHCP, standalone flux install, non-root kubeconfig, and clean cluster kustomization (#29)

### Summary of Changes

1. **Systemd-Networkd DHCP & Base System**:
- Added `/etc/systemd/network/10-eth.network` with `Name=eth* en*` and
`DHCP=yes`.
- Enabled `systemd-networkd` and `systemd-resolved` so all ethernet
interfaces get DHCP automatically on first boot.
   - Added `git` package to base image.

2. **Flux v2 Standalone Installation**:
- Replaced `kubectl apply -k` with standalone `kubectl apply -f
https://github.com/fluxcd/flux2/releases/download/v2.4.0/install.yaml`
so Flux installs in 2 seconds without git dependencies.

3. **Kubectl Access for Non-Root Users**:
- Copies `/etc/rancher/k3s/k3s.yaml` into `/home/<user>/.kube/config`
with `0600` permissions and proper user ownership.

4. **Cluster Kustomization Cleanliness**:
- Removed self-referential `- flux-system` from
`ci/k8s/clusters/production/kustomization.yaml`.

ci/k8s/clusters/production/kustomization.yaml

@@ -2,7 +2,6 @@ apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 
 resources:
-  - flux-system
   - ../../base/namespaces
   - ../../infrastructure/cert-manager
   - ../../infrastructure/networking