feat(FN-4678): add removeWorktree helper to worktree-backend

Adds a `removeWorktree` helper to the worktree backend with corresponding unit tests, and wires it into the worktree pool.

Fusion-Task-Id: FN-4678

packages/engine/src/worktree-pool.ts

@@ -14,6 +14,7 @@ export {
   NativeWorktreeBackend,
   WorktrunkOperationError,
   WorktrunkWorktreeBackend,
+  removeWorktree,
   resolveWorktreeBackend,
 } from "./worktree-backend.js";
 export type { WorktreeBackend, WorktreeBackendKind } from "./worktree-backend.js";