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/Migrations/20260427130058_RetireLegacyTaskStatus.cs

@@ -0,0 +1,51 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace ClaudeDo.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class RetireLegacyTaskStatus : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            // manual / draft -> idle
+            migrationBuilder.Sql("UPDATE tasks SET status = 'idle' WHERE status IN ('manual', 'draft');");
+
+            // planning -> idle + planning_phase=active
+            migrationBuilder.Sql("UPDATE tasks SET status = 'idle', planning_phase = 'active' WHERE status = 'planning';");
+
+            // planned -> idle + planning_phase=finalized
+            migrationBuilder.Sql("UPDATE tasks SET status = 'idle', planning_phase = 'finalized' WHERE status = 'planned';");
+
+            // waiting -> queued + blocked_by_task_id derived from sort_order chain.
+            // SQLite 3.25+ supports window functions (LAG).
+            migrationBuilder.Sql(@"
+WITH ordered AS (
+  SELECT id,
+         LAG(id) OVER (PARTITION BY parent_task_id ORDER BY sort_order, created_at) AS prev_id
+  FROM tasks
+  WHERE status = 'waiting'
+)
+UPDATE tasks
+SET status = 'queued',
+    blocked_by_task_id = (SELECT prev_id FROM ordered WHERE ordered.id = tasks.id)
+WHERE id IN (SELECT id FROM ordered);");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            // Best-effort and lossy: cancelled is folded back into failed,
+            // (idle, finalized) -> planned, (idle, active) -> planning,
+            // queued + blocked_by_task_id != null -> waiting.
+            // Manual/Draft distinction is unrecoverable — anything previously
+            // 'manual' or 'draft' stays 'idle' on the way back.
+            migrationBuilder.Sql("UPDATE tasks SET status = 'failed' WHERE status = 'cancelled';");
+            migrationBuilder.Sql("UPDATE tasks SET status = 'planned' WHERE status = 'idle' AND planning_phase = 'finalized';");
+            migrationBuilder.Sql("UPDATE tasks SET status = 'planning' WHERE status = 'idle' AND planning_phase = 'active';");
+            migrationBuilder.Sql("UPDATE tasks SET status = 'waiting', blocked_by_task_id = NULL WHERE status = 'queued' AND blocked_by_task_id IS NOT NULL;");
+        }
+    }
+}