feat(KB-332): rename task prefix from KB to FN and branches from kb/ to fusion/

- Change default task prefix from KB to FN across all packages

- Rename branch naming pattern from kb/{id} to fusion/{id}

- Update all test assertions and fixtures to use new prefixes

- Add changeset for the breaking change

- Resolve merge conflicts in TaskCard.test.tsx touch gesture tests

packages/engine/src/worktree-pool.ts

@@ -111,8 +111,8 @@ export class WorktreePool {
    * 3. `git checkout -B <branchName> <startPoint>` — create/reset branch from start point
    *
    * @param worktreePath — Absolute path to the recycled worktree
-   * @param branchName — Branch name for the new task (e.g., `kb/kb-042`)
-   * @param startPoint — Git ref to branch from (e.g., `kb/kb-041`). Defaults to `main`.
+   * @param branchName — Branch name for the new task (e.g., `fusion/fn-042`)
+   * @param startPoint — Git ref to branch from (e.g., `fusion/fn-041`). Defaults to `main`.
    */
   prepareForTask(worktreePath: string, branchName: string, startPoint?: string): void {
     // Clean tracked modifications