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/PlanningSessionManagerTests.cs

@@ -67,7 +67,7 @@ public sealed class PlanningSessionManagerTests : IDisposable
             ListId = listId,
             Title = "Brainstorm auth",
             Description = "- review tokens\n- plan rollout",
-            Status = TaskStatus.Manual,
+            Status = TaskStatus.Idle,
             CreatedAt = DateTime.UtcNow,
             CommitType = "feat",
         };
@@ -101,7 +101,8 @@ public sealed class PlanningSessionManagerTests : IDisposable
         Assert.Contains("review tokens", initial);
 
         var loaded = await _tasks.GetByIdAsync(parent.Id);
-        Assert.Equal(TaskStatus.Planning, loaded!.Status);
+        Assert.Equal(TaskStatus.Idle, loaded!.Status);
+        Assert.Equal(PlanningPhase.Active, loaded.PlanningPhase);
         Assert.NotNull(loaded.PlanningSessionToken);
     }
 
@@ -220,7 +221,8 @@ public sealed class PlanningSessionManagerTests : IDisposable
 
         Assert.False(Directory.Exists(startCtx.Files.SessionDirectory));
         var loaded = await _tasks.GetByIdAsync(parent.Id);
-        Assert.Equal(TaskStatus.Manual, loaded!.Status);
+        Assert.Equal(TaskStatus.Idle, loaded!.Status);
+        Assert.Equal(PlanningPhase.None, loaded.PlanningPhase);
         Assert.Null(loaded.PlanningSessionToken);
     }