feat(headless): establish portable Linux host boundary

Add the presentation-free acdream-headless executable, strict no-connect configuration validation, dependency and assembly guards, and a Windows/Ubuntu CI lane that builds and tests only the portable runtime closure.

Co-authored-by: Codex <noreply@openai.com>

src/AcDream.Headless/Configuration/HeadlessConfigurationException.cs

@@ -0,0 +1,9 @@
+namespace AcDream.Headless.Configuration;
+
+internal sealed class HeadlessConfigurationException : Exception
+{
+    internal HeadlessConfigurationException(string message)
+        : base(message)
+    {
+    }
+}