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/Repositories/TaskRepositoryTests.cs

@@ -163,7 +163,7 @@ public sealed class TaskRepositoryTests : IDisposable
         using var readCtx = _db.CreateContext();
         var after = await new TaskRepository(readCtx).GetByIdAsync(task.Id);
         Assert.NotNull(after);
-        Assert.Equal(TaskStatus.Manual, after!.Status);
+        Assert.Equal(TaskStatus.Idle, after!.Status);
         Assert.Null(after.StartedAt);
         Assert.Null(after.FinishedAt);
         Assert.Null(after.Result);