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.

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -117,7 +117,7 @@ public sealed class ExternalMcpService
             ListId = listId,
             Title = title,
             Description = description,
-            Status = TaskStatus.Manual,
+            Status = TaskStatus.Idle,
             CreatedAt = DateTime.UtcNow,
             CommitType = list.DefaultCommitType,
             CreatedBy = createdBy,
@@ -167,7 +167,7 @@ public sealed class ExternalMcpService
         return ToDto(reload);
     }
 
-    [McpServerTool, Description("Update a task's status. Only 'Manual' and 'Queued' are permitted — use RunTaskNow or CancelTask for execution control.")]
+    [McpServerTool, Description("Update a task's status. Only 'Idle' and 'Queued' are permitted — use RunTaskNow or CancelTask for execution control.")]
     public async Task<TaskDto> UpdateTaskStatus(
         string taskId,
         string status,
@@ -181,7 +181,7 @@ public sealed class ExternalMcpService
 
         switch (target)
         {
-            case TaskStatus.Manual:
+            case TaskStatus.Idle:
                 await _tasks.ResetToManualAsync(taskId, cancellationToken);
                 await _broadcaster.TaskUpdated(taskId);
                 break;