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/HeadlessExitCode.cs

@@ -0,0 +1,8 @@
+namespace AcDream.Headless;
+
+internal enum HeadlessExitCode
+{
+    Success = 0,
+    UsageError = 2,
+    ConfigurationError = 3,
+}