feat(nixos): add settings option for builder config

Allow defining builder repository configuration directly in Nix using
the `settings.repos` option, which is more idiomatic for NixOS modules.

Users can now choose between:
- `settings.repos` - Define repos in Nix (recommended)
- `configFile` - Point to an external YAML file

The module generates a YAML config file from settings when configFile
is not specified. An assertion ensures at least one method is used.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -322,14 +322,47 @@ Default `deploySubjects`:
 | `package` | package | from flake | Package to use |
 | `natsUrl` | string | required | NATS server URL |
 | `nkeyFile` | path | required | Path to NKey seed file |
-| `configFile` | path | required | Path to builder configuration file |
+| `configFile` | path | `null` | Path to builder config file (alternative to `settings`) |
+| `settings.repos` | attrs | `{}` | Repository configuration (see below) |
 | `timeout` | int | `1800` | Build timeout per host in seconds |
 | `environment` | attrs | `{}` | Additional environment variables |
 | `metrics.enable` | bool | `false` | Enable Prometheus metrics endpoint |
 | `metrics.address` | string | `":9973"` | Metrics HTTP server address |
 | `metrics.openFirewall` | bool | `false` | Open firewall for metrics port |
 
-Example builder configuration:
+Each entry in `settings.repos` is an attribute set with:
+
+| Option | Type | Default | Description |
+|--------|------|---------|-------------|
+| `url` | string | required | Git flake URL (must start with `git+https://`, `git+ssh://`, or `git+file://`) |
+| `defaultBranch` | string | `"master"` | Default branch to build when not specified |
+
+Example builder configuration using `settings`:
+
+```nix
+services.homelab-deploy.builder = {
+  enable = true;
+  natsUrl = "nats://nats.example.com:4222";
+  nkeyFile = "/run/secrets/homelab-deploy-builder-nkey";
+  settings.repos = {
+    nixos-servers = {
+      url = "git+https://git.example.com/org/nixos-servers.git";
+      defaultBranch = "master";
+    };
+    homelab = {
+      url = "git+ssh://git@github.com/user/homelab.git";
+      defaultBranch = "main";
+    };
+  };
+  metrics = {
+    enable = true;
+    address = ":9973";
+    openFirewall = true;
+  };
+};
+```
+
+Alternatively, you can use `configFile` to point to an external YAML file:
 
 ```nix
 services.homelab-deploy.builder = {
@@ -337,11 +370,6 @@ services.homelab-deploy.builder = {
   natsUrl = "nats://nats.example.com:4222";
   nkeyFile = "/run/secrets/homelab-deploy-builder-nkey";
   configFile = "/etc/homelab-deploy/builder.yaml";
-  metrics = {
-    enable = true;
-    address = ":9973";
-    openFirewall = true;
-  };
 };
 ```