feat(FN-4044): restore github tracking patch persistence

Restored GitHub tracking patch persistence in the core store with matching test coverage in the task detail and routes tasks ops suites, plus documentation for the repaired flow.

Fusion-Task-Id: FN-4044

packages/core/src/__tests__/store-github-tracking.test.ts

@@ -52,6 +52,23 @@ describe("TaskStore github tracking", () => {
     });
   });
 
+  it("persists githubTracking through generic updateTask patch flow", async () => {
+    const task = await store.createTask({ description: "Patch issue" });
+
+    await store.updateTask(task.id, {
+      githubTracking: {
+        enabled: true,
+        repoOverride: "octocat/hello-world",
+      },
+    });
+
+    const updated = await store.getTask(task.id);
+    expect(updated?.githubTracking).toEqual({
+      enabled: true,
+      repoOverride: "octocat/hello-world",
+    });
+  });
+
   it("links and unlinks tracked issue while preserving other tracking fields", async () => {
     const task = await store.createTask({ description: "Link issue" });