feat(worker): PlanningSessionManager.StartAsync

Add PlanningSessionFiles, PlanningSessionStartContext/ResumeContext DTOs,
PlanningSessionManager.StartAsync (file scaffolding + status transition),
and integration tests. Also fix migration discovery by adding [DbContext]
attribute to all migration classes and switch DbFixture to EnsureCreated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Worker/Planning/PlanningSessionFiles.cs

@@ -0,0 +1,7 @@
+namespace ClaudeDo.Worker.Planning;
+
+public sealed record PlanningSessionFiles(
+    string SessionDirectory,
+    string McpConfigPath,
+    string SystemPromptPath,
+    string InitialPromptPath);