feat(worktree): base improvement-child worktree on parent HEAD

src/ClaudeDo.Worker/Runner/WorktreeManager.cs

@@ -32,7 +32,7 @@ public sealed class WorktreeManager
         if (!await _git.IsGitRepoAsync(workingDir, ct))
             throw new InvalidOperationException($"working_dir is not a git repository: {workingDir}");
 
-        var baseCommit = await _git.RevParseHeadAsync(workingDir, ct);
+        var baseCommit = await ResolveBaseCommitAsync(task, workingDir, ct);
         // Use the full task id (dashes stripped) in the branch name so
         // two GUIDs sharing an 8-char prefix cannot collide on the same branch.
         var idForBranch = task.Id.Replace("-", "");
@@ -184,4 +184,25 @@ public sealed class WorktreeManager
 
         _logger.LogInformation("Discarded worktree for task {TaskId} (branch {Branch})", wt.TaskId, wt.BranchName);
     }
+
+    // Improvement children (parent is a non-planning task with its own worktree) branch
+    // from the parent's recorded HEAD so they build on the parent's not-yet-merged work.
+    // Planning children and standalone tasks base off the list's current HEAD.
+    private async Task<string> ResolveBaseCommitAsync(TaskEntity task, string workingDir, CancellationToken ct)
+    {
+        if (task.ParentTaskId is not null)
+        {
+            using var ctx = _dbFactory.CreateDbContext();
+            var parent = await ctx.Tasks.AsNoTracking()
+                .FirstOrDefaultAsync(t => t.Id == task.ParentTaskId, ct);
+            if (parent is not null && parent.PlanningPhase == PlanningPhase.None)
+            {
+                var parentWt = await new WorktreeRepository(ctx).GetByTaskIdAsync(task.ParentTaskId, ct);
+                var parentHead = parentWt?.HeadCommit ?? parentWt?.BaseCommit;
+                if (parentHead is not null)
+                    return parentHead;
+            }
+        }
+        return await _git.RevParseHeadAsync(workingDir, ct);
+    }
 }