fix: implement cancel endpoint and pool cancel mechanism

POST /api/tasks/{id}/cancel now works. Pool tracks a cancel func per
running task ID; Cancel(taskID) calls it and returns false if the task
isn't running. The execute goroutine registers/deregisters the cancel
func around the runner call.

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

1 files changed, 14 insertions, 0 deletions

diff --git a/internal/api/server.go b/internal/api/server.go
index 5758347..18c58e9 100644
--- a/internal/api/server.go
+++ b/internal/api/server.go
@@ -61,6 +61,7 @@ func (s *Server) routes() {
 	s.mux.HandleFunc("GET /api/tasks", s.handleListTasks)
 	s.mux.HandleFunc("GET /api/tasks/{id}", s.handleGetTask)
 	s.mux.HandleFunc("POST /api/tasks/{id}/run", s.handleRunTask)
+	s.mux.HandleFunc("POST /api/tasks/{id}/cancel", s.handleCancelTask)
 	s.mux.HandleFunc("POST /api/tasks/{id}/accept", s.handleAcceptTask)
 	s.mux.HandleFunc("POST /api/tasks/{id}/reject", s.handleRejectTask)
 	s.mux.HandleFunc("GET /api/tasks/{id}/subtasks", s.handleListSubtasks)
@@ -109,6 +110,19 @@ func (s *Server) BroadcastQuestion(taskID, toolUseID string, questionData json.R
 	s.hub.Broadcast(data)
 }
 
+func (s *Server) handleCancelTask(w http.ResponseWriter, r *http.Request) {
+	taskID := r.PathValue("id")
+	if _, err := s.store.GetTask(taskID); err != nil {
+		writeJSON(w, http.StatusNotFound, map[string]string{"error": "task not found"})
+		return
+	}
+	if !s.pool.Cancel(taskID) {
+		writeJSON(w, http.StatusConflict, map[string]string{"error": "task is not running"})
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]string{"status": "cancelling"})
+}
+
 func (s *Server) handleAnswerQuestion(w http.ResponseWriter, r *http.Request) {
 	taskID := r.PathValue("id")