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/PlanningSessionContext.cs

@@ -0,0 +1,12 @@
+namespace ClaudeDo.Worker.Planning;
+
+public sealed record PlanningSessionStartContext(
+    string ParentTaskId,
+    string WorkingDir,
+    PlanningSessionFiles Files);
+
+public sealed record PlanningSessionResumeContext(
+    string ParentTaskId,
+    string WorkingDir,
+    string ClaudeSessionId,
+    string McpConfigPath);