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/PlanningChainCoordinator.cs

@@ -39,16 +39,10 @@ public sealed class PlanningChainCoordinator
         if (children.Count == 0)
             throw new InvalidOperationException("Parent has no subtasks.");
 
-        // Eligibility: new layout uses Status=Idle. Tolerate legacy Manual/Planned/Draft
-        // values during this slice — they will be migrated away in slice 6.
-        var bad = children.FirstOrDefault(c =>
-            c.Status != TaskStatus.Idle &&
-            c.Status != TaskStatus.Manual &&
-            c.Status != TaskStatus.Planned &&
-            c.Status != TaskStatus.Draft);
+        var bad = children.FirstOrDefault(c => c.Status != TaskStatus.Idle);
         if (bad is not null)
             throw new InvalidOperationException(
-                $"Child {bad.Id} is in status {bad.Status}; expected Idle (or legacy Manual/Planned/Draft).");
+                $"Child {bad.Id} is in status {bad.Status}; expected Idle.");
 
         // Worker queue picker requires the "agent" tag — attach it so children are pickable.
         var agentTag = await ctx.Tags.FirstOrDefaultAsync(t => t.Name == "agent", ct);