nix: add runner images outputs

docs/RUNNER_IMAGES.md

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+# Runner Images (NixOS)
+
+This repo exports reproducible NixOS runner configurations via flake outputs:
+
+- `nixosConfigurations.ec-runner-aarch64`
+- `nixosConfigurations.ec-runner-x86_64`
+- `nixosConfigurations.ec-runner-aarch64-netboot`
+- `nixosConfigurations.ec-runner-x86_64-netboot`
+- `nixosConfigurations.ec-runner-x86_64-iso`
+- `nixosConfigurations.ec-runner-aarch64-sdimage`
+
+The runner OS exposes this repo's flake source inside the system at:
+
+- `/etc/every-channel/flake`
+
+This allows a runner to self-build and verify artifacts from the same flake definition.
+
+## Build (OrbStack / Linux)
+
+These commands should be run inside a Linux environment with Nix enabled (e.g. OrbStack VM).
+
+Build netboot artifacts (iPXE/PXE):
+
+```sh
+nix build .#nixosConfigurations.ec-runner-aarch64-netboot.config.system.build.netboot
+```
+
+Build an installer ISO (x86_64):
+
+```sh
+nix build .#nixosConfigurations.ec-runner-x86_64-iso.config.system.build.isoImage
+```
+
+Build an aarch64 SD image:
+
+```sh
+nix build .#nixosConfigurations.ec-runner-aarch64-sdimage.config.system.build.sdImage
+```
+
+## Outputs
+
+After building, artifacts will be in `./result` (a symlink into the Nix store).
+
+Common netboot outputs include:
+
+- `kernel`
+- `initrd`
+- `netboot.ipxe`
+
+Exact filenames may vary across NixOS releases.