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/HeadlessConfiguration.cs

@@ -0,0 +1,18 @@
+using System.Text.Json.Serialization;
+
+namespace AcDream.Headless.Configuration;
+
+internal sealed class HeadlessConfiguration
+{
+    [JsonRequired]
+    public int Version { get; init; }
+
+    [JsonRequired]
+    public List<HeadlessSessionDescriptor?> Sessions { get; init; } = [];
+}
+
+internal sealed class HeadlessSessionDescriptor
+{
+    [JsonRequired]
+    public string Id { get; init; } = string.Empty;
+}