docs: add missing AGENTS.md (and README where missing)

AGENTS.md

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+# AGENTS.md
+
+Guidance for coding agents working in this repository.
+
+## What this repo is
+
+`deploy-sourcegraph-k8s` holds Kustomize manifests for deploying Sourcegraph
+on Kubernetes. It is primarily declarative YAML, with a few small Go helper
+tools used in CI. There is no application server to build here.
+
+> Note: this Kustomize deployment method is planned for deprecation in favor of
+> Helm. See `README.md`.
+
+## Where the code lives
+
+- `base/` — base Kustomize resources (`kubectl kustomize ./base`), including `base/monitoring` and `base/sourcegraph`.
+- `components/` — reusable Kustomize components (sizes, storage-class, network, ingress, executors, etc.) layered onto overlays.
+- `examples/` — ready-made overlays per environment/provider (aws, gke, k3s, minikube, demo, …).
+- `instances/` — where users create their own overlay; copy from `instances/template`.
+- `.buildkite/` — CI pipeline (`pipeline.yaml`), verification shell scripts, and Go tools under `.buildkite/tools` and `.buildkite/verify-release`.
+
+## Toolchain
+
+Tool versions are pinned in `.tool-versions` (managed via mise/asdf): Go 1.21.5,
+kustomize 4.5.7, kubectl 1.25.9, shfmt 3.1.0, Node 20.8.1, yarn 1.22.4,
+github-cli 2.46.0.
+
+## Common commands
+
+Render/validate manifests:
+
+```bash
+# render the base
+kubectl kustomize ./base/.
+
+# render an example overlay (e.g. gke)
+kubectl kustomize examples/gke
+```
+
+CI checks (run from the repo root; mirror `.buildkite/pipeline.yaml`):
+
+```bash
+.buildkite/verify-yaml.sh          # kustomize build + kubectl dry-run apply (needs cluster creds)
+.buildkite/verify-label.sh         # every base manifest must have metadata.labels.deploy == sourcegraph
+.buildkite/verify-rbac-labels.sh   # RBAC label checks
+.buildkite/verify-overlays.sh      # kustomize-render every overlay under examples/
+.buildkite/check-image-names.sh    # validate image names in base manifests
+.buildkite/verify-release/verify-release.sh
+```
+
+Go helper tools (have unit tests):
+
+```bash
+cd .buildkite/tools/check-image-names && go test ./...
+```
+
+## Conventions
+
+- Every resource manifest under `base/` (except `kustomization.yaml`) must carry the label `metadata.labels.deploy: sourcegraph` — enforced by `verify-label.sh`.
+- Add new overlays under `examples/<name>/` or `instances/<name>/`; `verify-overlays.sh` will render each one.
+- Format shell scripts with `shfmt` (version pinned in `.tool-versions`).
+- Releases are driven by branch naming and `release.yaml` / `sg release` in CI; do not change branch-name patterns in `.buildkite/pipeline.yaml` without updating release tooling.