feat(KB-334): change default task prefix from KB to FN

- Change default taskPrefix from KB to FN and branch naming from kb/ to fusion/
- Update all test expectations and patterns for new naming convention
- Update settings UI placeholder text to reflect FN prefix
- Update dashboard, engine, and CLI components for fusion/ branch paths
- Add changeset documenting the breaking change for users

packages/dashboard/app/components/SettingsModal.tsx

@@ -454,7 +454,7 @@ export function SettingsModal({
               <input
                 id="taskPrefix"
                 type="text"
-                placeholder="KB"
+                placeholder="FN"
                 value={form.taskPrefix || ""}
                 onChange={(e) => {
                   const val = e.target.value;

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

@@ -32,7 +32,7 @@ describe("MergeDetails", () => {
             deletions: 2,
             mergedAt: "2026-01-01T01:00:00.000Z",
             prNumber: 42,
-            mergeCommitMessage: "feat(KB-001): merge kb/kb-001",
+            mergeCommitMessage: "feat(FN-001): merge fusion/fn-001",
             mergeConfirmed: true,
           },
         })}
@@ -44,7 +44,7 @@ describe("MergeDetails", () => {
     expect(screen.getByText("5")).toBeTruthy();
     expect(screen.getByText("+10 / -2")).toBeTruthy();
     expect(screen.getByText("#42")).toBeTruthy();
-    expect(screen.getByText("feat(KB-001): merge kb/kb-001")).toBeTruthy();
+    expect(screen.getByText("feat(FN-001): merge fusion/fn-001")).toBeTruthy();
     expect(screen.getByText("Merged successfully")).toBeTruthy();
   });
 });