fix: add missing import and update tests for branch template feature (#799)

* fix: add missing import and update tests for branch template feature

- Add missing `import { $ } from 'bun'` in branch.ts
- Add missing `labels` property to pull-request-target.test.ts fixture
- Update branch-template tests to expect 5-word descriptions

* address review feedback: update comment and add truncation test

test/pull-request-target.test.ts

@@ -87,6 +87,7 @@ describe("pull_request_target event support", () => {
       },
       comments: { nodes: [] },
       reviews: { nodes: [] },
+      labels: { nodes: [] },
     },
     comments: [],
     changedFiles: [],