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.Data/Configuration/TaskEntityConfiguration.cs

@@ -17,12 +17,6 @@ public class TaskEntityConfiguration : IEntityTypeConfiguration<TaskEntity>
             TaskStatus.Done      => "done",
             TaskStatus.Failed    => "failed",
             TaskStatus.Cancelled => "cancelled",
-            // Legacy values — kept for compat until slice 6 retires them.
-            TaskStatus.Manual    => "manual",
-            TaskStatus.Planning  => "planning",
-            TaskStatus.Planned   => "planned",
-            TaskStatus.Draft     => "draft",
-            TaskStatus.Waiting   => "waiting",
             _ => throw new ArgumentOutOfRangeException(nameof(v)),
         };
 
@@ -35,12 +29,6 @@ public class TaskEntityConfiguration : IEntityTypeConfiguration<TaskEntity>
             "done"      => TaskStatus.Done,
             "failed"    => TaskStatus.Failed,
             "cancelled" => TaskStatus.Cancelled,
-            // Legacy values — kept for compat until slice 6 retires them.
-            "manual"    => TaskStatus.Manual,
-            "planning"  => TaskStatus.Planning,
-            "planned"   => TaskStatus.Planned,
-            "draft"     => TaskStatus.Draft,
-            "waiting"   => TaskStatus.Waiting,
             _ => throw new ArgumentOutOfRangeException(nameof(v)),
         };