FN-5665: allow editing mission target branch

Enable mission target branch to be viewed and updated from the mission edit form.

- Add `baseBranch` to mission form state and initialize it when opening edit mode.
- Include trimmed `baseBranch` in mission PATCH updates.
- Add target branch input to both MissionManager edit form render paths.
- Add MissionManager tests for pre-filling and saving edited target branch values.

Files changed:
 .../dashboard/app/components/MissionManager.tsx    | 24 ++++++++
 .../components/__tests__/MissionManager.test.tsx   | 66 ++++++++++++++++++++++
 2 files changed, 90 insertions(+)

Fusion-Task-Id: FN-5665
Fusion-Task-Lineage: b25cf171-b681-4dd8-b5f4-70f0e8714774

packages/dashboard/app/components/MissionManager.tsx

@@ -253,6 +253,7 @@ interface MissionFormData {
   description: string;
   status: MissionStatus;
   autopilotEnabled: boolean;
+  baseBranch: string;
 }
 
 interface MilestoneFormData {
@@ -281,6 +282,7 @@ const EMPTY_MISSION_FORM: MissionFormData = {
   description: "",
   status: "planning",
   autopilotEnabled: false,
+  baseBranch: "",
 };
 
 const EMPTY_MILESTONE_FORM: MilestoneFormData = {
@@ -1407,6 +1409,7 @@ export function MissionManager({ isOpen, isInline = false, onClose, addToast, pr
       description: mission.description || "",
       status: mission.status,
       autopilotEnabled: mission.autopilotEnabled ?? false,
+      baseBranch: mission.baseBranch ?? "",
     });
   }, []);
 
@@ -1439,6 +1442,7 @@ export function MissionManager({ isOpen, isInline = false, onClose, addToast, pr
           description: missionForm.description.trim() || undefined,
           status: missionForm.status,
           autopilotEnabled: missionForm.autopilotEnabled,
+          baseBranch: missionForm.baseBranch.trim() || "",
         };
         if (missionForm.autopilotEnabled) {
           updates.autoAdvance = true;
@@ -2436,6 +2440,16 @@ export function MissionManager({ isOpen, isInline = false, onClose, addToast, pr
                     onChange={(e) => setMissionForm({ ...missionForm, description: e.target.value })}
                     rows={2}
                   />
+                  <label>
+                    Target branch
+                    <input
+                      type="text"
+                      placeholder="e.g. main"
+                      value={missionForm.baseBranch}
+                      onChange={(e) => setMissionForm({ ...missionForm, baseBranch: e.target.value })}
+                      aria-label="Mission target branch"
+                    />
+                  </label>
                   <div className="mission-form-card__row">
                     <select
                       value={missionForm.status}
@@ -4083,6 +4097,16 @@ export function MissionManager({ isOpen, isInline = false, onClose, addToast, pr
                     onChange={(e) => setMissionForm({ ...missionForm, description: e.target.value })}
                     rows={2}
                   />
+                  <label>
+                    Target branch
+                    <input
+                      type="text"
+                      placeholder="e.g. main"
+                      value={missionForm.baseBranch}
+                      onChange={(e) => setMissionForm({ ...missionForm, baseBranch: e.target.value })}
+                      aria-label="Mission target branch"
+                    />
+                  </label>
                   <div className="mission-form-card__row">
                     <select
                       value={missionForm.status}

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

@@ -2923,6 +2923,72 @@ describe("MissionManager", () => {
       });
     });
 
+    it("pre-fills target branch when editing a mission", async () => {
+      const missionDetailWithBranch = {
+        ...mockMissionDetail,
+        baseBranch: "develop",
+      };
+      globalThis.fetch = createDetailFetchMockForMissionDetail(missionDetailWithBranch);
+
+      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 waitForDetailLoaded();
+
+      fireEvent.click(screen.getAllByLabelText("Edit mission")[0]);
+
+      const targetBranchInput = await screen.findByLabelText("Mission target branch");
+      expect(targetBranchInput).toHaveValue("develop");
+    });
+
+    it("saves edited target branch via mission patch", async () => {
+      const fetchMock = vi.fn().mockImplementation((url: string, init?: RequestInit) => {
+        if (url.includes("/health")) {
+          const missionId = extractMissionId(url) ?? "M-001";
+          return Promise.resolve(mockApiResponse(getMockMissionHealth(missionId)));
+        }
+        if (url === "/api/missions" || url.includes("/api/missions?")) {
+          return Promise.resolve(mockApiResponse(mockMissions));
+        }
+        if (url === "/api/missions/M-001" && (!init?.method || init.method === "GET")) {
+          return Promise.resolve(mockApiResponse(mockMissionDetail));
+        }
+        if (url === "/api/missions/M-001" && init?.method === "PATCH") {
+          return Promise.resolve(mockApiResponse({ ...mockMissionDetail, baseBranch: "release/2026.05" }));
+        }
+        return Promise.resolve(mockApiResponse({}));
+      });
+      globalThis.fetch = fetchMock;
+
+      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 waitForDetailLoaded();
+
+      fireEvent.click(screen.getAllByLabelText("Edit mission")[0]);
+
+      const targetBranchInput = await screen.findByLabelText("Mission target branch");
+      fireEvent.change(targetBranchInput, { target: { value: "  release/2026.05  " } });
+      fireEvent.click(screen.getByRole("button", { name: "Update" }));
+
+      await waitFor(() => {
+        const patchCall = fetchMock.mock.calls.find(
+          (call) => call[0] === "/api/missions/M-001" && call[1]?.method === "PATCH",
+        );
+        expect(patchCall).toBeDefined();
+        const body = JSON.parse((patchCall![1] as RequestInit).body as string);
+        expect(body.baseBranch).toBe("release/2026.05");
+      });
+    });
+
     it("shows delete confirmation with danger variant class", async () => {
       globalThis.fetch = createDetailFetchMock();
       render(<MissionManager isOpen={true} onClose={vi.fn()} addToast={vi.fn()} />);