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
+
+This is the **deployment reference for Sourcegraph with Docker Compose** (`github.com/sourcegraph/deploy-sourcegraph-docker`). It is primarily configuration (Docker Compose YAML, service configs, dashboards) plus small Go and shell tooling for testing and releases — not an application codebase.
+
+The `main` branch tracks development. Versioned branches (e.g. `3.19`) correspond to released Sourcegraph versions.
+
+## Where the main code/config lives
+
+- `docker-compose/` — the deployment reference; `docker-compose/docker-compose.yaml` is the primary entrypoint, with `dev/`, `examples/`, `executors/`, and `jaeger/` variants.
+- Service config directories: `caddy/`, `nginx/`, `pgsql/`, `codeintel-db/`, `codeinsights-db/`, `grafana/`, `prometheus/`, `otel-collector/`.
+- `test/` — smoke and upgrade tests (`smoke-test.sh`, `upgrade-test.go`, `Vagrantfile`, `servers.yaml`).
+- `tools/` — Go-pinned CLIs (`update-docker-tags`) and `migrate.sh`.
+- `.buildkite/` — CI scripts and pipeline.
+
+## Toolchain versions
+
+Defined in `.tool-versions` (asdf/mise): Go `1.19.8`, Node `16.7.0`, Yarn `1.22.4`, shellcheck `0.7.1`, github-cli `2.46.0`, checkov.
+
+## Setup, build, and checks
+
+These mirror what CI (`.buildkite/pipeline.yaml`) actually runs.
+
+Install JS dev dependencies (for prettier):
+
+```bash
+yarn
+```
+
+Validate the Docker Compose config:
+
+```bash
+cd docker-compose
+docker-compose -f docker-compose.yaml config -q
+```
+
+Or via the CI wrapper from the repo root:
+
+```bash
+.buildkite/validate-docker-compose.sh
+```
+
+Format / check formatting (prettier, config in `prettier.config.js`):
+
+```bash
+yarn run prettier        # write
+yarn run prettier-check  # check only
+```
+
+Run the Terraform/IaC security scan (Checkov):
+
+```bash
+.buildkite/ci-checkov.sh
+```
+
+## Testing
+
+Smoke and upgrade tests run on GCP via Vagrant (see `TESTING.md`):
+
+```bash
+.buildkite/vagrant-run.sh docker-test   # TEST_TYPE=docker-compose-test
+```
+
+This starts a GCP instance, uploads the repo, and runs the docker-compose smoke test. Expected output: `docker-test: ALL TESTS PASSED`.
+
+## Conventions
+
+- Keep changes minimal and config-focused; this is a deployment reference consumed by customers.
+- All `*.{js,json,ts,tsx,graphql,md,scss}` files are formatted with prettier — run `yarn run prettier` before committing.
+- Shell scripts should pass shellcheck.
+- Releases are driven by `release.yaml` and the `sg release` steps in `.buildkite/pipeline.yaml`; release branches use `internal/release-*` and `promote/release-*` naming.