feat(FN-1456): add explicit abandon flow for planning and subtask AI sessions

- Add abandon action to useBackgroundSessions hook for deterministic session cleanup
- Make background dismissal deterministic instead of relying on 7-day TTL expiration
- Update PlanningModeModal to include abandon button in header actions
- Add comprehensive tests for abandon behavior and session lifecycle
- Update dashboard guide with session lifecycle documentation

packages/dashboard/app/components/PlanningModeModal.test.tsx

@@ -207,6 +207,7 @@ describe("PlanningModeModal", () => {
     mockAcquireSessionLock.mockResolvedValue({ acquired: true, currentHolder: null });
     mockReleaseSessionLock.mockResolvedValue(undefined);
     mockForceAcquireSessionLock.mockResolvedValue(undefined);
+    mockCancelPlanning.mockResolvedValue(undefined);
 
     // Default: simulate receiving a question after a brief delay
     mockConnectPlanningStream.mockImplementation((_sessionId: string, _projectId: string | undefined, handlers: any) => {
@@ -1530,7 +1531,7 @@ describe("PlanningModeModal", () => {
       expect(mockOnClose).toHaveBeenCalled();
     });
 
-    it("closes active question session without canceling server session", async () => {
+    it("closes active question session and abandons server session", async () => {
       const confirmSpy = vi.spyOn(window, "confirm");
 
       render(
@@ -1554,11 +1555,12 @@ describe("PlanningModeModal", () => {
       fireEvent.click(screen.getByLabelText("Close"));
 
       expect(confirmSpy).not.toHaveBeenCalled();
-      expect(mockCancelPlanning).not.toHaveBeenCalled();
+      // Closing an active session should abandon it on the server
+      expect(mockCancelPlanning).toHaveBeenCalledTimes(1);
       expect(mockOnClose).toHaveBeenCalled();
     });
 
-    it("closes summary view without canceling server session", async () => {
+    it("closes summary view and abandons server session", async () => {
       const confirmSpy = vi.spyOn(window, "confirm");
 
       mockConnectPlanningStream.mockImplementationOnce((_sessionId: string, _projectId: string | undefined, handlers: any) => {
@@ -1593,11 +1595,12 @@ describe("PlanningModeModal", () => {
       fireEvent.click(screen.getByLabelText("Close"));
 
       expect(confirmSpy).not.toHaveBeenCalled();
-      expect(mockCancelPlanning).not.toHaveBeenCalled();
+      // Closing an active session should abandon it on the server
+      expect(mockCancelPlanning).toHaveBeenCalledTimes(1);
       expect(mockOnClose).toHaveBeenCalled();
     });
 
-    it("closes via overlay without canceling server session", async () => {
+    it("closes via overlay and abandons server session", async () => {
       const confirmSpy = vi.spyOn(window, "confirm");
 
       const { container } = render(
@@ -1623,11 +1626,12 @@ describe("PlanningModeModal", () => {
       fireEvent.click(overlay!);
 
       expect(confirmSpy).not.toHaveBeenCalled();
-      expect(mockCancelPlanning).not.toHaveBeenCalled();
+      // Closing an active session should abandon it on the server
+      expect(mockCancelPlanning).toHaveBeenCalledTimes(1);
       expect(mockOnClose).toHaveBeenCalled();
     });
 
-    it("closes during loading state without canceling server session", async () => {
+    it("closes during loading state and abandons server session", async () => {
       const confirmSpy = vi.spyOn(window, "confirm");
 
       mockConnectPlanningStream.mockImplementationOnce(() => ({
@@ -1656,7 +1660,8 @@ describe("PlanningModeModal", () => {
       fireEvent.click(screen.getByLabelText("Close"));
 
       expect(confirmSpy).not.toHaveBeenCalled();
-      expect(mockCancelPlanning).not.toHaveBeenCalled();
+      // Closing an active session should abandon it on the server
+      expect(mockCancelPlanning).toHaveBeenCalledTimes(1);
       expect(mockOnClose).toHaveBeenCalled();
     });
 
@@ -1693,7 +1698,7 @@ describe("PlanningModeModal", () => {
       expect(mockOnClose).toHaveBeenCalledTimes(1);
     });
 
-    it("disconnects SSE stream on close", async () => {
+    it("disconnects SSE stream and abandons session on close", async () => {
       const closeSpy = vi.fn();
 
       mockConnectPlanningStream.mockImplementationOnce(() => ({
@@ -1722,7 +1727,8 @@ describe("PlanningModeModal", () => {
       fireEvent.click(screen.getByLabelText("Close"));
 
       expect(closeSpy).toHaveBeenCalledTimes(1);
-      expect(mockCancelPlanning).not.toHaveBeenCalled();
+      // Closing an active session should abandon it on the server
+      expect(mockCancelPlanning).toHaveBeenCalledTimes(1);
       expect(mockOnClose).toHaveBeenCalled();
     });
   });

packages/dashboard/app/components/PlanningModeModal.tsx

@@ -11,6 +11,7 @@ import {
   startPlanningBreakdown,
   createTasksFromPlanning,
   fetchModels,
+  cancelPlanning,
   type PlanningSession,
   type SubtaskItem,
   type ModelInfo,
@@ -467,6 +468,17 @@ export function PlanningModeModal({ isOpen, onClose, onTaskCreated, onTasksCreat
   }, [onClose]);
 
   const handleCancel = useCallback(() => {
+    // Determine the active session ID to abandon
+    let activeSessionId: string | null = null;
+    if (view.type === "question" || view.type === "summary" || view.type === "error") {
+      activeSessionId = view.session.sessionId;
+    } else if (view.type === "breakdown") {
+      activeSessionId = view.sessionId;
+    } else if (view.type === "loading") {
+      // During loading, the session ID is stored in the ref
+      activeSessionId = currentSessionIdRef.current;
+    }
+
     // Save to localStorage BEFORE any cleanup (preserve for re-entry)
     if (initialPlan) {
       savePlanningDescription(initialPlan, projectId);
@@ -476,6 +488,13 @@ export function PlanningModeModal({ isOpen, onClose, onTaskCreated, onTasksCreat
     streamConnectionRef.current?.close();
     streamConnectionRef.current = null;
 
+    // Explicitly abandon the session on the server to prevent zombie sessions
+    if (activeSessionId) {
+      void cancelPlanning(activeSessionId, projectId, sessionTabId).catch(() => {
+        // Best-effort: cancellation failures should not block UI reset
+      });
+    }
+
     setInitialPlan("");
     setView({ type: "initial" });
     setError(null);
@@ -490,7 +509,7 @@ export function PlanningModeModal({ isOpen, onClose, onTaskCreated, onTasksCreat
     currentSessionIdRef.current = null;
     setLockSessionId(null);
     onClose();
-  }, [initialPlan, onClose, projectId]);
+  }, [initialPlan, onClose, projectId, sessionTabId, view]);
 
   // Handle escape key to close
   useEffect(() => {