Introduce agents.md

Signed-off-by: Adam Warner <github@promofaux.dev>

Author: PromoFaux

AGENTS.md

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+# AGENTS.md
+
+## Project overview
+
+The official Pi-hole Docker image. It packages FTL, the core scripts and the web interface into a single container, with startup logic that translates environment variables into Pi-hole configuration.
+
+## Repository layout
+
+- `src/Dockerfile` - the image definition
+- `src/start.sh` - container entrypoint
+- `src/bash_functions.sh` - startup logic (environment variable handling, configuration conversion)
+- `src/crontab.txt` - scheduled in-container jobs
+- `build.sh` - local image build helper
+- `test/` - BATS test suite and `test/run.sh` harness
+- `examples/` - example compose files and deployment configurations
+
+## Dev environment tips
+
+- You need Docker and git; everything builds and runs in containers.
+- Build a local image with `./build.sh`.
+- The image is published for multiple architectures; avoid amd64-only assumptions in the Dockerfile or scripts.
+
+## Testing instructions
+
+- Run the suite with `bash test/run.sh`. It builds `pihole:test`, starts one container per configuration under test, runs BATS against them, and cleans up on exit.
+- See `test/TESTING.md` for details, including testing other platforms via emulation: `CIPLATFORM=linux/arm64 bash test/run.sh`
+- Any change to startup behaviour or environment variable handling needs a corresponding BATS test.
+
+## PR instructions
+
+- Base all work on the `development` branch; pull requests target `development`.
+- Read the [contributors guide](https://docs.pi-hole.net/guides/github/contributing/)
+- Every commit must be signed off (DCO): use `git commit -s`.
+- Run `bash test/run.sh` before committing.
+- Use Unix line endings (LF); shell scripts in this image run under the container's shell, not Windows.
+- Code is licensed under the EUPL 1.2; contributions must be compatible.
+- Environment variables are the public interface of this image. Document any new or changed variable in the README, and keep backwards compatibility unless a break is explicitly agreed.
+- Stability comes before features; this image runs unattended on many systems.
+- The correct project spelling is "Pi-hole" (capital P, lowercase h, hyphen).
+
+## Security considerations
+
+- Secrets can be supplied via Docker secrets as well as environment variables; never log their values during startup.
+- Be deliberate about container capabilities and dropped privileges; do not add capability requirements without discussion.
+- If you believe you have found a vulnerability, do not open a public issue or PR; report it privately per the organisation's security policy (disclosure@pi-hole.net).
+
+## Common pitfalls
+
+- Changing environment variable behaviour without updating the README and tests.
+- Adding packages to the image without considering size and multi-arch availability.
+- Forgetting the DCO sign-off on commits.
+- Assuming host paths or Docker Desktop specifics; the image must work on plain Linux Docker and compose.