ci: fork-safe checkout for lint workflow (#5887)

Use conditional ref to support forks and avoid checkout failures.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5887-ci-fork-safe-checkout-for-lint-workflow-2806d73d36508139b9effa6d18e1cadb)
by [Unito](https://www.unito.io)

.github/workflows/lint-and-format.yaml

@@ -15,7 +15,7 @@ jobs:
       - name: Checkout PR
         uses: actions/checkout@v5
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ !github.event.pull_request.head.repo.fork && github.head_ref || github.ref }}
 
       - name: Install pnpm
         uses: pnpm/action-setup@v4
@@ -100,4 +100,4 @@ jobs:
               owner: context.repo.owner,
               repo: context.repo.repo,
               body: '## ⚠️ Linting/Formatting Issues Found\n\nThis PR has linting or formatting issues that need to be fixed.\n\n**Since this PR is from a fork, auto-fix cannot be applied automatically.**\n\n### Option 1: Set up pre-commit hooks (recommended)\nRun this once to automatically format code on every commit:\n```bash\npnpm prepare\n```\n\n### Option 2: Fix manually\nRun these commands and push the changes:\n```bash\npnpm lint:fix\npnpm format\n```\n\nSee [CONTRIBUTING.md](https://github.com/Comfy-Org/ComfyUI_frontend/blob/main/CONTRIBUTING.md#git-pre-commit-hooks) for more details.'
-            })
+            })