feat: add AGENTS.md

AGENTS.md

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+# Repository Guidelines
+
+## Project Structure & Module Organization
+- `hosts/<host>/configuration.nix` defines each machine; host modules, packages, and site configs live alongside for composability.
+- Shared building blocks sit in `utils/` (`modules/`, `overlays/`, `pkgs/`, `bento.nix`), while `fleet.nix` centralizes cross-host user provisioning.
+- Provisioning assets (ISO profiles, Raspberry Pi imaging, helper scripts) live under `iso/`, `raspberry*/`, and `scripts/`—refer to them before reinventing steps.
+
+## Build, Test, and Development Commands
+- Enter the dev shell via `nix-shell` (uses `shell.nix`) to populate MCP helper configs and standard tooling.
+- Dry-run any change with `./scripts/test-configuration <host>`; append `-v` to mirror `nixos-rebuild --show-trace` for deeper diagnostics.
+- Deployment relies on the Git runner—once reviewed changes merge to main, the runner rebuilds and switches the relevant host automatically; treat a clean dry-run as the gate before pushing.
+
+## Coding Style & Naming Conventions
+- Format Nix files with two-space indentation; run `nixpkgs-fmt` (via `nix run nixpkgs#nixpkgs-fmt .`) before committing complex edits.
+- Keep module and derivation names in lower kebab-case (`web-arm`, `home-assistant.nix`) and align attribute names with actual host or service identifiers.
+- Use comments sparingly to justify non-obvious decisions (open ports, unusual service options) and prefer explicit imports over wildcard includes.
+
+## Testing Guidelines
+- Always run `./scripts/test-configuration <host>` before raising a PR; it ensures evaluation succeeds and secrets are present.
+- For service changes, confirm activation with `nixos-rebuild test` (or `switch`) on a staging machine and capture any notable logs.
+- Document manual smoke checks (e.g., URLs defined in `hosts/web-arm/sites/`) in the PR so reviewers can repeat them quickly.
+
+## Commit & Pull Request Guidelines
+- Follow the Conventional Commits pattern used in `git log` (`fix:`, `chore:`, `update:`) and scope by host when helpful (`fix(mail):`).
+- Split refactors, secrets rotations, and package bumps into distinct commits to simplify review and rollback.
+- PRs should call out affected hosts, link dry-build output (and confirm the runner result after merge), and tag the owners noted in `hosts/<host>/users/*.nix`; attach screenshots for UI-facing updates.
+
+## Security & Configuration Tips
+- Configure `config.sh` before provisioning SFTP users so the values consumed by `fleet.nix` stay in sync with the chroot layout.
+- Store API keys referenced in `shell.nix` (such as the Brave Search token) under `~/.config/mcp-servers/` and keep real secrets out of version control.
+- Rotate and edit encrypted `hosts/<host>/secrets.yaml` via `nix-shell -p sops --run 'sops hosts/<host>/secrets.yaml'`; commit only the encrypted output.