feat(storage): emit subtask_spawned events on the parent stream

CreateTask is now transactional and, when a task has a parent_task_id,
records a subtask_spawned event (actor agent) on the parent's stream so
the parent's history shows the children it dispatched. Top-level task
creation (no parent) emits nothing.

This closes the one agent-originated signal that flows during a run today
(subtasks POSTed to the API per the planning preamble) but previously
left no trace in the event stream.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

1 files changed, 29 insertions, 3 deletions

diff --git a/internal/storage/db.go b/internal/storage/db.go
index a871c71..adb7c02 100644
--- a/internal/storage/db.go
+++ b/internal/storage/db.go
@@ -190,15 +190,41 @@ func (s *DB) CreateTask(t *task.Task) error {
 		return fmt.Errorf("marshaling depends_on: %w", err)
 	}
 
-	_, err = s.db.Exec(`
+	tx, err := s.db.Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback() //nolint:errcheck
+
+	if _, err = tx.Exec(`
 		INSERT INTO tasks (id, name, description, elaboration_input, project, repository_url, config_json, priority, timeout_ns, retry_json, tags_json, depends_on_json, parent_task_id, state, created_at, updated_at, story_id, acceptance_criteria, checker_for_task_id, checker_report)
 		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
 		t.ID, t.Name, t.Description, t.ElaborationInput, t.Project, t.RepositoryURL, string(configJSON), string(t.Priority),
 		t.Timeout.Duration.Nanoseconds(), string(retryJSON), string(tagsJSON), string(depsJSON),
 		t.ParentTaskID, string(t.State), t.CreatedAt.UTC(), t.UpdatedAt.UTC(), t.StoryID,
 		t.AcceptanceCriteria, t.CheckerForTaskID, t.CheckerReport,
-	)
-	return err
+	); err != nil {
+		return err
+	}
+
+	// A subtask records a subtask_spawned event on its parent's stream so the
+	// parent's history shows the children it dispatched.
+	if t.ParentTaskID != "" {
+		payload, _ := json.Marshal(struct {
+			SubtaskID string `json:"subtask_id"`
+			Name      string `json:"name"`
+		}{SubtaskID: t.ID, Name: t.Name})
+		if err = createEventTx(tx, &event.Event{
+			TaskID:  t.ParentTaskID,
+			Kind:    event.KindSubtaskSpawned,
+			Actor:   event.ActorAgent,
+			Payload: payload,
+		}); err != nil {
+			return err
+		}
+	}
+
+	return tx.Commit()
 }
 
 // GetTask retrieves a task by ID.