feat: Create a basic AGENTS.md file for LLMS (#160)

### Summary

Create an [AGENTS.md](https://agents.md/) file. Uses some examples:

- https://github.com/apache/airflow/blob/main/AGENTS.md
- https://github.com/apache/superset/blob/master/AGENTS.md

Preferred AGENTS.md over LLMS.md as it is a more commonly used format,
e.g. in tools like CodeRabbit.ai

Fixes #154 

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Added comprehensive guide documentation covering repository overview,
development setup, testing procedures, deployment guidance, contribution
standards, and troubleshooting resources.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: martyngigg

AGENTS.md

@@ -0,0 +1,100 @@
+# LLM Context Guide for Analytics Data Platform
+
+This repository contains ELT code for an Iceberg-based data warehouse, together with
+infrastructure code used to run the warehouse both locally (docker-compose) and in an
+OpenStack cloud (Ansible).
+
+Use this document as a concise, developer-friendly reference when working with the
+repository or providing context to an assistant/agent about the project structure and
+common developer tasks.
+
+## Purpose
+
+- Give human developers and LLM-based agents the essential repository layout and
+  pointers for local development, testing and deployment.
+- Point to the configuration locations used for certificates, local environments,
+  and deployable Ansible playbooks/roles.
+
+## Repository layout (high level)
+
+```text
+.
+├── certs/                  # Certificate request configurations used for HTTPS/SSL
+├── elt-common/             # Reusable Python package with common ETL/ELT helpers used by the warehouses
+├── docs/                   # User and developer documentation using MkDocs. See `docs/src` for content used in the published docs site.
+├── infra/
+│   ├── ansible-docker/     # Ansible playbooks/roles to deploy the system to the STFC (OpenStack) cloud.
+│   └── local/              # docker-compose configuration for a local development environment and end-to-end CI tests.
+└── warehouses/             # One subdirectory per (Lakekeeper) warehouse. Each contains ELT code to extract, transform and load data from external sources into Iceberg tables.
+```
+
+## Pre-commit hooks
+
+- Static code checks are enforced using `prek`. Install with your environment's tooling and run checks
+  on all files with `prek run --all-files`. Install the pre-commit hook with `prek install`.
+
+## Running tests
+
+- There are both unit and e2e tests for the `elt-common` package under `elt-common/tests/`. They are
+  written using `pytest`.
+- Install the package in editable mode + dev dependencies in a Python environment, e.g. with `uv`
+
+```bash
+cd elt-common/
+uv venv
+uv install . --editable --group dev
+```
+
+- Run unit tests with:
+
+```bash
+cd elt-common/
+uv run pytest unit_tests
+```
+
+- The e2e tests require a running data platform stack. Use the docker-based setup in `infra/local`
+  to start the stack, run the tests and bring down the stack:
+
+```bash
+pushd infra/local; docker compose up -d; popd    # start the services
+cd elt-common/
+uv run pytest e2e_tests
+pushd infra/local; docker compose down -v; popd  # stop the services
+```
+
+## Cloud deployment
+
+- Use the Ansible playbooks in `infra/ansible-docker/` together with the
+  `inventory*.yml` files. See the `infra/ansible-docker/readme.md` for role and
+  variable guidance.
+
+## Pull Request Guidelines
+
+- When creating pull requests:
+
+  1. **Read the current PR template**: Always check `.github/PULL_REQUEST_TEMPLATE.md` for the latest format
+  2. **Follow PR title conventions**: Use [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/)
+     - Format: `type(scope): description`
+     - Example: `fix(warehouse/accelerator): fix join in model`
+     - Types: `fix`, `feat`, `docs`, `style`, `refactor`, `perf`, `test`, `chore`
+
+**Important**: Always reference the actual template file at `.github/PULL_REQUEST_TEMPLATE.md` instead of using cached content, as the template may be updated over time.
+
+## Troubleshooting & tips
+
+- Docker resource issues: the local compose stack can be resource heavy. Ensure
+  Docker Desktop has enough CPU/memory.
+- Ansible role errors: ensure you have required galaxy roles (see
+  `infra/ansible-docker/ansible-galaxy-requirements.yaml`) and the correct Python
+  and Ansible versions installed.
+
+## Where to go next
+
+- Read `docs/` and `docs-devel/` for high-level architecture and deployment
+  instructions.
+- Inspect `warehouses/` for per-warehouse ELT implementations and examples.
+
+---
+
+This file was created to provide a concise, shareable context document for humans
+and LLM-based agents working with the analytics-data-platform monorepo.