feat(worker): add PlanningChainCoordinator for sequential subtask execution

Coordinates Waiting -> Queued transitions between sibling subtasks: when a child finishes Done, the next Waiting sibling is promoted to Queued. WorkerHub.QueuePlanningSubtasksAsync exposes this to the UI; TaskRunner advances the chain on completion. Also tightens the planning-session prompt: planner must use MCP tools, not direct edits.

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

src/ClaudeDo.Worker/Runner/TaskRunner.cs

@@ -3,7 +3,9 @@ using ClaudeDo.Data.Models;
 using ClaudeDo.Data.Repositories;
 using ClaudeDo.Worker.Config;
 using ClaudeDo.Worker.Hub;
+using ClaudeDo.Worker.Planning;
 using Microsoft.EntityFrameworkCore;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
 
 namespace ClaudeDo.Worker.Runner;
 
@@ -16,6 +18,7 @@ public sealed class TaskRunner
     private readonly ClaudeArgsBuilder _argsBuilder;
     private readonly WorkerConfig _cfg;
     private readonly ILogger<TaskRunner> _logger;
+    private readonly PlanningChainCoordinator _chain;
 
     public TaskRunner(
         IClaudeProcess claude,
@@ -24,7 +27,8 @@ public sealed class TaskRunner
         WorktreeManager wtManager,
         ClaudeArgsBuilder argsBuilder,
         WorkerConfig cfg,
-        ILogger<TaskRunner> logger)
+        ILogger<TaskRunner> logger,
+        PlanningChainCoordinator chain)
     {
         _claude = claude;
         _dbFactory = dbFactory;
@@ -33,6 +37,7 @@ public sealed class TaskRunner
         _argsBuilder = argsBuilder;
         _cfg = cfg;
         _logger = logger;
+        _chain = chain;
     }
 
     public async Task RunAsync(TaskEntity task, string slot, CancellationToken ct)
@@ -338,6 +343,23 @@ public sealed class TaskRunner
         await _broadcaster.TaskFinished(slot, task.Id, "done", finishedAt);
         _logger.LogInformation("Task {TaskId} completed (turns={Turns}, tokens_in={In}, tokens_out={Out})",
             task.Id, result.TurnCount, result.TokensIn, result.TokensOut);
+
+        // Sequential planning chain: if this task has a parent, flip the next
+        // Waiting sibling to Queued so the queue pickup loop dispatches it next.
+        if (task.ParentTaskId is not null)
+        {
+            try
+            {
+                var advanced = await _chain.OnChildFinishedAsync(
+                    task.Id, TaskStatus.Done, CancellationToken.None);
+                if (advanced is not null)
+                    await _broadcaster.TaskUpdated(advanced);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogWarning(ex, "PlanningChain advance failed for {TaskId}", task.Id);
+            }
+        }
     }
 
     private async Task HandleFailure(string taskId, string taskTitle, string slot, RunResult result)