refactor(data): retire legacy TaskStatus values and backfill existing rows

Slice 6 of the worker state and queue consolidation refactor.

* Drop Manual, Planning, Planned, Draft, Waiting from the TaskStatus enum
  and from the EF value converter; only the lifecycle values remain
  (Idle, Queued, Running, Done, Failed, Cancelled).
* Add migration RetireLegacyTaskStatus that rewrites existing rows:
  manual/draft -> idle, planning -> idle+planning_phase=active,
  planned -> idle+planning_phase=finalized, waiting -> queued+blocked_by
  derived from sort_order via a CTE with LAG().
* Reroute every call site that compared/set legacy values to the new
  three-field model (Status + PlanningPhase + BlockedByTaskId), including
  the planning repo helpers, MCP services, the planning chain coordinator,
  and the UI view-models. TaskRowViewModel now exposes PlanningPhase to
  drive the planning badge.
* Refresh Worker/CLAUDE.md and Data/CLAUDE.md, the docs/plan.md status
  section, and the planning verification notes in docs/open.md.

tests/ClaudeDo.Worker.Tests/Planning/PlanningAggregatorTests.cs

@@ -67,7 +67,7 @@ public class PlanningAggregatorTests : IDisposable
         ctx.Tasks.Add(new TaskEntity
         {
             Id = parentId, ListId = listId, Title = "plan", CreatedAt = DateTime.UtcNow,
-            Status = TaskStatus.Planning, SortOrder = 0,
+            Status = TaskStatus.Idle, PlanningPhase = PlanningPhase.Active, SortOrder = 0,
         });
 
         // Two children (sorted A then B).
@@ -171,7 +171,7 @@ public class PlanningAggregatorTests : IDisposable
         ctx.Tasks.Add(new TaskEntity
         {
             Id = parentId, ListId = listId, Title = "plan", CreatedAt = DateTime.UtcNow,
-            Status = TaskStatus.Planning, SortOrder = 0,
+            Status = TaskStatus.Idle, PlanningPhase = PlanningPhase.Active, SortOrder = 0,
         });
         var subA = Guid.NewGuid().ToString();
         var subB = Guid.NewGuid().ToString();