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>

.gitignore

@@ -0,0 +1,13 @@
+# IDE
+.idea/
+.vscode/
+*.iml
+
+# Go build artifacts
+/bin/
+*.exe
+*.test
+*.out
+
+# OS
+.DS_Store