refactor(worker/state): introduce TaskStateService and route mutations through it

Slice 2 of the worker state consolidation refactor (spec sections 2 and 8).

Adds Worker/State/ITaskStateService + TaskStateService as the single component
that mutates Status, PlanningPhase, and BlockedByTaskId. Each transition is one
atomic ExecuteUpdate with a WHERE filter on the expected source status, so
parallel claims are TOCTOU-free. Side effects (queue wake on -> Queued, hub
TaskUpdated broadcast, chain advance + parent completion on terminal child)
are owned by the service so callers no longer need to remember them.

Migrated callers (mechanical, behavior preserved):
- TaskRunner: HandleSuccess/HandleFailure/MarkFailed/RunAsync/ContinueAsync
- StaleTaskRecovery: bulk recover stale Running tasks
- TaskResetService: status flip (worktree cleanup stays in service)
- PlanningSessionManager.StartAsync: status flip via state, token write via repo
- PlanningChainCoordinator.OnChildFinishedAsync: routes the next-sibling write
  through state.UnblockAsync (Slice 4 finishes the rewrite)
- ExternalMcpService.UpdateTaskStatus: Queued case via state.EnqueueAsync

Repo Mark*Async helpers (MarkRunning/MarkDone/MarkFailed/FlipAllRunningToFailed)
are now internal; ClaudeDo.Data grants InternalsVisibleTo to ClaudeDo.Worker
and ClaudeDo.Worker.Tests for the existing repo-level tests.

DI: TaskStateService is registered as Singleton in both the main app and the
external-MCP app; the queue-wake delegate captures sp -> QueueService.WakeQueue
to break the TaskStateService -> QueueService -> TaskRunner -> TaskStateService
construction cycle. PlanningChainCoordinator takes Func<ITaskStateService> for
the same reason; Slice 3 will replace both with IQueueWaker.

Tests: TaskStateServiceTests covers happy + reject for every transition, the
parallel StartRunningAsync claim race, child-terminal chain advancement, and
stale recovery. Existing service/repo tests are updated to construct the new
state-service via a TaskStateServiceBuilder helper. Pre-existing constructor
drift in QueueService/ExternalMcp/PlanningHub tests is patched to keep the
test project building (the surrounding test logic is otherwise untouched).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Worker/Planning/PlanningChainCoordinator.cs

@@ -1,5 +1,6 @@
 using ClaudeDo.Data;
 using ClaudeDo.Data.Models;
+using ClaudeDo.Worker.State;
 using Microsoft.EntityFrameworkCore;
 using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
 
@@ -8,9 +9,15 @@ namespace ClaudeDo.Worker.Planning;
 public sealed class PlanningChainCoordinator
 {
     private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
+    private readonly Func<ITaskStateService> _state;
 
-    public PlanningChainCoordinator(IDbContextFactory<ClaudeDoDbContext> dbFactory)
-        => _dbFactory = dbFactory;
+    public PlanningChainCoordinator(
+        IDbContextFactory<ClaudeDoDbContext> dbFactory,
+        Func<ITaskStateService> state)
+    {
+        _dbFactory = dbFactory;
+        _state = state;
+    }
 
     public async Task QueueSubtasksSequentiallyAsync(string parentTaskId, CancellationToken ct = default)
     {
@@ -56,6 +63,7 @@ public sealed class PlanningChainCoordinator
         if (child?.ParentTaskId is null) return null;
 
         var next = await ctx.Tasks
+            .AsNoTracking()
             .Where(t => t.ParentTaskId == child.ParentTaskId
                      && t.SortOrder > child.SortOrder
                      && t.Status == TaskStatus.Waiting)
@@ -63,8 +71,7 @@ public sealed class PlanningChainCoordinator
             .FirstOrDefaultAsync(ct);
         if (next is null) return null;
 
-        next.Status = TaskStatus.Queued;
-        await ctx.SaveChangesAsync(ct);
+        await _state().UnblockAsync(next.Id, ct);
         return next.Id;
     }
 }