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>

tests/ClaudeDo.Worker.Tests/Infrastructure/DbFixture.cs

@@ -10,9 +10,9 @@ public sealed class DbFixture : IDisposable
     public DbFixture()
     {
         DbPath = Path.Combine(Path.GetTempPath(), $"claudedo_test_{Guid.NewGuid():N}.db");
-        // Apply migrations so the schema is created.
+        // EnsureCreated uses the current model directly — no Designer.cs needed.
         using var ctx = CreateContext();
-        ctx.Database.Migrate();
+        ctx.Database.EnsureCreated();
     }
 
     public ClaudeDoDbContext CreateContext()