feat(worker): fast-forward finished children into the parent's unit branch

Planning parents get their own worktree on Queue plan, children fork from the
unit head and are absorbed via merge --ff-only before Done (ff failure -> Failed,
interactive child approve -> absorb instead of merge to the target).

src/ClaudeDo.Worker/Planning/PlanningChainCoordinator.cs

@@ -1,5 +1,6 @@
 using ClaudeDo.Data;
 using ClaudeDo.Data.Models;
+using ClaudeDo.Worker.Runner;
 using ClaudeDo.Worker.State;
 using Microsoft.EntityFrameworkCore;
 using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
@@ -10,13 +11,16 @@ public sealed class PlanningChainCoordinator
 {
     private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
     private readonly Func<ITaskStateService> _state;
+    private readonly UnitBranchService? _unitBranch;
 
     public PlanningChainCoordinator(
         IDbContextFactory<ClaudeDoDbContext> dbFactory,
-        Func<ITaskStateService> state)
+        Func<ITaskStateService> state,
+        UnitBranchService? unitBranch = null)
     {
         _dbFactory = dbFactory;
         _state = state;
+        _unitBranch = unitBranch;
     }
 
     // Sets up a sequential chain over a planning parent's children.
@@ -86,6 +90,9 @@ public sealed class PlanningChainCoordinator
         if (phase != PlanningPhase.Finalized)
             throw new InvalidOperationException("Plan must be finalized before it can be queued.");
 
+        // The parent's worktree is the unit branch every child forks from and is absorbed into.
+        if (_unitBranch is not null)
+            await _unitBranch.EnsureUnitWorktreeAsync(parentTaskId, ct);
         return await SetupChainAsync(parentTaskId, enqueue: true, ct);
     }