feat: initial implementation of gateway-cert-operator

Kubernetes operator that automates HTTPS listener configuration on
Gateway API Gateway resources whenever a cert-manager Certificate is
created or updated.

Core behaviour:
- Watches cert-manager Certificate resources for the annotation
  gateway-cert-operator.io/gateway-name to identify the target Gateway
- Builds HTTPS listeners (prefixed "auto-") from each Certificate's
  DNS SANs and merges them into the target Gateway's listener list
- Preserves any manually-managed listeners; removes stale auto-listeners
  when Certificates are deleted or their annotations are removed
- Supports optional annotations to override the target namespace and
  listener port (default 443)

Components:
- main.go                            – manager setup, scheme registration,
                                       health/readiness probes
- internal/controller/               – Certificate reconciler with field
                                       indexing and dual-watch pattern
- internal/gateway/patch.go          – listener construction, merge, and
                                       equality helpers
- deploy/manifests.yaml              – Namespace, RBAC, and Deployment
- docs/README.md                     – usage guide and architecture notes
- Dockerfile                         – distroless multi-stage build

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

deploy/manifests.yaml

@@ -0,0 +1,102 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: gateway-cert-operator-system
+---
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: gateway-cert-operator
+  namespace: gateway-cert-operator-system
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRole
+metadata:
+  name: gateway-cert-operator
+rules:
+  # Watch and read Certificates
+  - apiGroups: ["cert-manager.io"]
+    resources: ["certificates"]
+    verbs: ["get", "list", "watch"]
+  # Read and patch Gateways
+  - apiGroups: ["gateway.networking.k8s.io"]
+    resources: ["gateways"]
+    verbs: ["get", "list", "watch", "patch"]
+  # Emit events
+  - apiGroups: [""]
+    resources: ["events"]
+    verbs: ["create", "patch"]
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRoleBinding
+metadata:
+  name: gateway-cert-operator
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: gateway-cert-operator
+subjects:
+  - kind: ServiceAccount
+    name: gateway-cert-operator
+    namespace: gateway-cert-operator-system
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: gateway-cert-operator
+  namespace: gateway-cert-operator-system
+  labels:
+    app.kubernetes.io/name: gateway-cert-operator
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: gateway-cert-operator
+  template:
+    metadata:
+      labels:
+        app.kubernetes.io/name: gateway-cert-operator
+    spec:
+      serviceAccountName: gateway-cert-operator
+      securityContext:
+        runAsNonRoot: true
+        seccompProfile:
+          type: RuntimeDefault
+      containers:
+        - name: manager
+          image: gateway-cert-operator:latest
+          args:
+            - --metrics-bind-address=:8080
+            - --health-probe-bind-address=:8081
+          ports:
+            - name: metrics
+              containerPort: 8080
+              protocol: TCP
+            - name: healthz
+              containerPort: 8081
+              protocol: TCP
+          livenessProbe:
+            httpGet:
+              path: /healthz
+              port: healthz
+            initialDelaySeconds: 5
+            periodSeconds: 10
+          readinessProbe:
+            httpGet:
+              path: /readyz
+              port: healthz
+            initialDelaySeconds: 5
+            periodSeconds: 10
+          resources:
+            limits:
+              cpu: 100m
+              memory: 64Mi
+            requests:
+              cpu: 50m
+              memory: 32Mi
+          securityContext:
+            allowPrivilegeEscalation: false
+            readOnlyRootFilesystem: true
+            capabilities:
+              drop: ["ALL"]