FN-5920: prefer mission detail event count for activity tab

Use mission detail event counts as the authoritative pre-load source for mission activity.

- add mission hierarchy eventCount support in core and dashboard mission types
- return mission eventCount from mission detail APIs and document the field
- prefer the detail eventCount over stale list summaries before activity events load
- add coverage for store, API, and MissionManager pre-load count behavior

Files changed:
 docs/missions.md                                   |  2 +-
 packages/core/src/__tests__/mission-store.test.ts  | 18 +++++++++
 packages/core/src/mission-store.ts                 |  6 +++
 packages/core/src/mission-types.ts                 |  2 +
 packages/dashboard/app/api/legacy.ts               |  2 +
 packages/dashboard/app/components/MissionManager.tsx    | 12 +++++-
 packages/dashboard/app/components/__tests__/MissionManager.test.tsx   | 46 +++++++++++++++++++++-
 packages/dashboard/app/components/mission-types.ts |  1 +
 packages/dashboard/src/__tests__/mission-e2e.test.ts    |  2 +
 9 files changed, 86 insertions(+), 5 deletions(-)

Fusion-Task-Id: FN-5920

Fusion-Task-Lineage: a0269499-d34a-495d-89fc-0534bb62a94e

packages/dashboard/app/components/MissionManager.tsx

@@ -922,8 +922,16 @@ export function MissionManager({ isOpen, isInline = false, onClose, addToast, pr
     if (!selectedMission?.id) {
       return eventsTotal;
     }
-    return missions.find((mission) => mission.id === selectedMission.id)?.summary?.eventCount ?? eventsTotal;
-  }, [eventsTotal, missions, selectedMission?.id]);
+
+    const baseCount = selectedMission.eventCount
+      ?? missions.find((mission) => mission.id === selectedMission.id)?.summary?.eventCount;
+
+    if (baseCount == null) {
+      return eventsTotal;
+    }
+
+    return Math.max(baseCount, eventsTotal);
+  }, [eventsTotal, missions, selectedMission?.eventCount, selectedMission?.id]);
 
   const displayedMissionEvents = useMemo(() => [...missionEvents].reverse(), [missionEvents]);
 

packages/dashboard/app/components/__tests__/MissionManager.test.tsx

@@ -132,6 +132,7 @@ const mockMissionDetail = {
   title: "Build Auth System",
   description: "Complete authentication flow",
   status: "planning",
+  eventCount: 4,
   linkedGoals: [] as Array<{ id: string; title: string; status: "active" | "archived"; createdAt: string; updatedAt: string; description?: string }>,
   milestones: [
     {
@@ -651,6 +652,8 @@ function createDetailFetchMockForMissionDetail(
   missionDetail: typeof mockMissionDetail,
   telemetryOverride: unknown = mockMilestoneValidationTelemetry,
   assertionsResponse: unknown[] = [],
+  missionsResponse = mockMissions,
+  eventsResponse = mockMissionEvents,
 ) {
   return vi.fn().mockImplementation((url: string) => {
     if (url.includes("/missions/health")) {
@@ -658,7 +661,7 @@ function createDetailFetchMockForMissionDetail(
     }
 
     if (url.includes("/events")) {
-      return Promise.resolve(mockApiResponse(parseMissionEventsResponse(url, mockMissionEvents)));
+      return Promise.resolve(mockApiResponse(parseMissionEventsResponse(url, eventsResponse)));
     }
 
     if (url.includes("/health")) {
@@ -686,7 +689,7 @@ function createDetailFetchMockForMissionDetail(
       }
     }
 
-    return Promise.resolve(mockApiResponse(mockMissions));
+    return Promise.resolve(mockApiResponse(missionsResponse));
   });
 }
 
@@ -1228,6 +1231,44 @@ describe("MissionManager", () => {
     });
   });
 
+  it("prefers mission detail event count when the list summary is stale before activity events load", async () => {
+    const staleSummaryMissions = mockMissions.map((mission) => mission.id === "M-001"
+      ? {
+        ...mission,
+        summary: {
+          ...mission.summary,
+          eventCount: 0,
+        },
+      }
+      : mission);
+    const missionDetailWithAuthoritativeCount = {
+      ...mockMissionDetail,
+      eventCount: 7,
+    };
+
+    globalThis.fetch = createDetailFetchMockForMissionDetail(
+      missionDetailWithAuthoritativeCount,
+      mockMilestoneValidationTelemetry,
+      [],
+      staleSummaryMissions,
+      [],
+    );
+    globalThis.EventSource = MockEventSource as unknown as typeof globalThis.EventSource;
+
+    render(<MissionManager isOpen={true} onClose={vi.fn()} addToast={vi.fn()} />);
+
+    await waitFor(() => {
+      expect(screen.getByText("Build Auth System")).toBeDefined();
+    });
+    fireEvent.click(screen.getByText("Build Auth System"));
+
+    await waitFor(() => {
+      expect(screen.getByTestId("mission-tab-activity")).toHaveTextContent("Activity (7)");
+    });
+
+    expect(screen.queryByTestId("mission-activity-events")).toBeNull();
+  });
+
   it("auto-scrolls to the latest mission activity on initial load", async () => {
     globalThis.fetch = createDetailFetchMock(mockMissionEvents);
     globalThis.EventSource = MockEventSource as unknown as typeof globalThis.EventSource;
@@ -1309,6 +1350,7 @@ describe("MissionManager", () => {
 
     await waitFor(() => {
       expect(screen.getByText("Real-time warning event")).toBeDefined();
+      expect(screen.getByTestId("mission-tab-activity")).toHaveTextContent("Activity (5)");
       expect(scrollIntoViewSpy).toHaveBeenLastCalledWith({ block: "end", behavior: "auto" });
     });
 

packages/dashboard/app/components/mission-types.ts

@@ -270,6 +270,7 @@ export type MissionWithSummary = Mission & { summary?: MissionSummary };
 export interface MissionWithHierarchy extends Mission {
   linkedGoals?: Goal[];
   milestones: Milestone[];
+  eventCount?: number;
 }
 
 /** Mission event categories emitted by mission observability APIs. */