Harden read-only provider with tests, docs, and CI

docs/data-sources/domain.md

@@ -0,0 +1,25 @@
+---
+page_title: "dynu_domain Data Source"
+description: |-
+  Resolves a hostname to its Dynu root domain and returns domain details.
+---
+
+# dynu_domain Data Source
+
+## Example Usage
+
+```terraform
+data "dynu_domain" "selected" {
+  hostname = "www.example.com"
+}
+```
+
+## Schema
+
+### Required
+
+- `hostname` (String) Fully-qualified hostname.
+
+### Read-Only
+
+- `domain` (Object) Same fields returned by `dynu_domains.domains[*]`.