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.Data/Migrations/20260422120000_AddTaskSortOrder.cs

@@ -1,3 +1,5 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable
@@ -5,6 +7,8 @@ using Microsoft.EntityFrameworkCore.Migrations;
 namespace ClaudeDo.Data.Migrations
 {
     /// <inheritdoc />
+    [DbContext(typeof(ClaudeDoDbContext))]
+    [Migration("20260422120000_AddTaskSortOrder")]
     public partial class AddTaskSortOrder : Migration
     {
         /// <inheritdoc />