docs: Campaign LA — pinned launch-contract schema COMMITTED into plan LA1

The LA3 Opus review process note was right: the contract both sides
implement lived only in orchestrator prompts, which is exactly the drift
mode the pin exists to prevent (and it produced the paths-key CRITICAL).
The schema, field rules, probe-mode discriminator, and status vocabulary
are now a binding plan section; amendments change this text first,
implementations second. Ledger: LA3 fix round dispatched.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Configuration/SessionConfigurationException.cs

@@ -0,0 +1,18 @@
+namespace AcDream.App.Configuration;
+
+/// <summary>Mirrors <c>AcDream.Headless.Configuration.HeadlessConfigurationException</c>
+/// — a semantic validation failure of an already well-typed session-config
+/// document (a type-SHAPE violation fails earlier, as a raw
+/// <see cref="System.Text.Json.JsonException"/> during deserialization).</summary>
+internal sealed class SessionConfigurationException : Exception
+{
+    internal SessionConfigurationException(string message)
+        : base(message)
+    {
+    }
+
+    internal SessionConfigurationException(string message, Exception innerException)
+        : base(message, innerException)
+    {
+    }
+}