executor: BLOCKED→READY for top-level tasks with subtasks

When a top-level task (ParentTaskID == "") finishes successfully,
check for subtasks before deciding the next state:
- subtasks exist → BLOCKED (waiting for subtasks to complete)
- no subtasks    → READY (existing behavior, unchanged)

This applies to both execute() and executeResume().

Adds ListSubtasks to the Store interface.

Tests:
- TestPool_Submit_TopLevel_WithSubtasks_GoesBlocked
- TestPool_Submit_TopLevel_NoSubtasks_GoesReady

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 64 insertions, 0 deletions

diff --git a/internal/executor/executor_test.go b/internal/executor/executor_test.go
index 1adba7e..91f7636 100644
--- a/internal/executor/executor_test.go
+++ b/internal/executor/executor_test.go
@@ -620,6 +620,70 @@ func TestPool_RateLimited_StaleEntryCleaned(t *testing.T) {
 	}
 }
 
+// TestPool_Submit_TopLevel_NoSubtasks_GoesReady verifies that a top-level task
+// with no subtasks still transitions to READY after successful execution.
+func TestPool_Submit_TopLevel_NoSubtasks_GoesReady(t *testing.T) {
+	store := testStore(t)
+	runner := &mockRunner{}
+	runners := map[string]Runner{"claude": runner}
+	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError}))
+	pool := NewPool(2, runners, store, logger)
+
+	tk := makeTask("no-subtasks-1") // no ParentTaskID, no subtasks
+	store.CreateTask(tk)
+
+	if err := pool.Submit(context.Background(), tk); err != nil {
+		t.Fatalf("submit: %v", err)
+	}
+
+	result := <-pool.Results()
+	if result.Err != nil {
+		t.Errorf("expected no error, got: %v", result.Err)
+	}
+	if result.Execution.Status != "READY" {
+		t.Errorf("status: want READY, got %q", result.Execution.Status)
+	}
+	got, _ := store.GetTask(tk.ID)
+	if got.State != task.StateReady {
+		t.Errorf("task state: want READY, got %v", got.State)
+	}
+}
+
+// TestPool_Submit_TopLevel_WithSubtasks_GoesBlocked verifies that when a
+// top-level task finishes successfully but has subtasks, it transitions to
+// BLOCKED (waiting for subtasks) rather than READY.
+func TestPool_Submit_TopLevel_WithSubtasks_GoesBlocked(t *testing.T) {
+	store := testStore(t)
+	runner := &mockRunner{}
+	runners := map[string]Runner{"claude": runner}
+	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelError}))
+	pool := NewPool(2, runners, store, logger)
+
+	parent := makeTask("parent-with-subtasks")
+	store.CreateTask(parent)
+
+	// Create a subtask in the store but do NOT submit it.
+	sub := makeTask("sub-of-parent")
+	sub.ParentTaskID = parent.ID
+	store.CreateTask(sub)
+
+	if err := pool.Submit(context.Background(), parent); err != nil {
+		t.Fatalf("submit: %v", err)
+	}
+
+	result := <-pool.Results()
+	if result.Err != nil {
+		t.Errorf("expected no error, got: %v", result.Err)
+	}
+	if result.Execution.Status != "BLOCKED" {
+		t.Errorf("status: want BLOCKED, got %q", result.Execution.Status)
+	}
+	got, _ := store.GetTask(parent.ID)
+	if got.State != task.StateBlocked {
+		t.Errorf("task state: want BLOCKED, got %v", got.State)
+	}
+}
+
 func TestPool_UnsupportedAgent(t *testing.T) {
 	store := testStore(t)
 	runners := map[string]Runner{"claude": &mockRunner{}}