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/Planning/PlanningSessionManager.cs

@@ -81,8 +81,9 @@ public sealed class PlanningSessionManager
             ?? throw new InvalidOperationException($"Task {taskId} not found.");
         if (task.ParentTaskId is not null)
             throw new InvalidOperationException("Cannot start a planning session on a child task.");
-        if (task.Status != TaskStatus.Manual)
-            throw new InvalidOperationException($"Task is in status {task.Status}; only Manual can start planning.");
+        if (task.Status != TaskStatus.Idle || task.PlanningPhase != PlanningPhase.None)
+            throw new InvalidOperationException(
+                $"Task is in status {task.Status}/{task.PlanningPhase}; only Idle+None can start planning.");
 
         var list = await lists.GetByIdAsync(task.ListId, ct)
             ?? throw new InvalidOperationException($"List {task.ListId} not found.");
@@ -232,7 +233,7 @@ public sealed class PlanningSessionManager
         var (tasks, _, settings, ctx) = CreateRepos();
         await using var __ = ctx;
         var children = await tasks.GetChildrenAsync(taskId, ct);
-        return children.Count(c => c.Status == TaskStatus.Draft);
+        return children.Count(c => c.Status == TaskStatus.Idle);
     }
 
     public async Task DiscardAsync(string taskId, CancellationToken ct)
@@ -261,8 +262,9 @@ public sealed class PlanningSessionManager
 
         var task = await tasks.GetByIdAsync(taskId, ct)
             ?? throw new InvalidOperationException($"Task {taskId} not found.");
-        if (task.Status != TaskStatus.Planning)
-            throw new InvalidOperationException($"Task is in status {task.Status}; resume requires Planning.");
+        if (task.PlanningPhase != PlanningPhase.Active)
+            throw new InvalidOperationException(
+                $"Task planning phase is {task.PlanningPhase}; resume requires Active planning.");
         if (string.IsNullOrEmpty(task.PlanningSessionId))
             throw new InvalidOperationException("No Claude session ID captured yet; cannot resume.");