Add AGENTS.md

This patch adds AGENTS.md to the repo to ensure any AI agent can follow
the conventions defined for this operator. At the same time, it allows
to not waste time navigating the repo to find initial information and
save tokens.

Signed-off-by: Francesco Pantano <fpantano@redhat.com>

Author: fmount

AGENTS.md

@@ -0,0 +1,79 @@
+# AGENTS.md - horizon-operator
+
+## Project overview
+
+See [README.md](README.md) for a full project overview, configuration examples,
+and contributing guidelines.
+
+## Tech stack
+
+| Layer | Technology |
+|-------|------------|
+| Language | Go (modules; `go.work` is local-only, never commit it) |
+| Scaffolding | [Kubebuilder v4](https://book.kubebuilder.io/) + [Operator SDK](https://sdk.operatorframework.io/) |
+| CRD generation | controller-gen (DeepCopy, CRDs, RBAC, webhooks) |
+| Config management | Kustomize |
+| Packaging | OLM bundle |
+| Testing | Ginkgo/Gomega + envtest (functional), KUTTL (integration) |
+| Linting | golangci-lint (`.golangci.yaml`) |
+| CI | Zuul (`zuul.d/`), Prow (`.ci-operator.yaml`), GitHub Actions |
+
+## Custom Resources
+
+| Kind | Purpose |
+|------|---------|
+| `Horizon` | Single CR managing the Horizon dashboard deployment (httpd/WSGI). No sub-CRs. |
+
+The `Horizon` CR has defaulting and validating admission webhooks.
+
+## Directory structure
+
+**Maintenance rule:** when directories are added, removed, or renamed, or when
+their purpose changes, update this table to match.
+
+| Directory | Contents |
+|-----------|----------|
+| `api/v1beta1/` | CRD types (`horizon_types.go`), conditions, webhook markers |
+| `cmd/` | `main.go` entry point |
+| `internal/controller/` | Reconciler: `horizon_controller.go` |
+| `internal/horizon/` | Horizon resource builders (deployment, volumes, config) |
+| `internal/webhook/` | Webhook implementation |
+| `templates/` | Config files and scripts mounted into pods via `OPERATOR_TEMPLATES` env var |
+| `config/crd,rbac,manager,webhook/` | Generated Kubernetes manifests (CRDs, RBAC, deployment, webhooks) |
+| `config/samples/` | Example CRs (Kustomize overlays). `base/`, `custom-theme/`, `httpd-overrides/` for customization. |
+| `test/functional/` | envtest-based Ginkgo/Gomega tests |
+| `test/kuttl/` | KUTTL integration tests |
+| `hack/` | Helper scripts (CRD schema checker, local webhook runner) |
+
+## Build commands
+
+After modifying Go code, always run: `make generate manifests fmt vet`.
+Before sending a commit, always run: `pre-commit run --all-files`.
+
+## Code style guidelines
+
+- Follow standard openstack-k8s-operators conventions and lib-common patterns.
+- Use `lib-common` modules for conditions, endpoints, TLS, storage, and other
+  cross-cutting concerns rather than re-implementing them.
+- CRD types go in `api/v1beta1/`. Controller logic goes in
+  `internal/controller/`. Resource-building helpers go in `internal/horizon/`.
+- Config templates are plain files in `templates/` -- they are mounted at
+  runtime via the `OPERATOR_TEMPLATES` environment variable.
+- Webhook logic is split between the kubebuilder markers in `api/v1beta1/` and
+  the implementation in `internal/webhook/`.
+
+## Testing
+
+- Functional tests use the envtest framework with Ginkgo/Gomega and live in
+  `test/functional/`.
+- KUTTL integration tests live in `test/kuttl/`.
+- Run all functional tests: `make test`.
+- When adding a new field or feature, add corresponding test cases in
+  `test/functional/` and update fixture data accordingly.
+
+## Key dependencies
+
+- [lib-common](https://github.com/openstack-k8s-operators/lib-common): shared modules for conditions, endpoints, TLS, secrets, etc.
+- [infra-operator](https://github.com/openstack-k8s-operators/infra-operator): Memcached and topology APIs.
+- [keystone-operator](https://github.com/openstack-k8s-operators/keystone-operator): identity service registration.
+- [dev-docs/developer.md](https://github.com/openstack-k8s-operators/dev-docs/blob/main/developer.md): developer guide and coding conventions.