ci: add GitHub Actions workflow for linting

Add a GitHub Actions workflow that runs linting and type checking for the
2025 directory using pnpm.

Author: ryoppippi

.github/workflows/lint.yml

@@ -0,0 +1,22 @@
+name: lint
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  lint-2025:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: ./2025
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - uses: pnpm/action-setup@v4
+        with:
+          run_install: false
+          version: 10.4.0
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm check
+      - run: pnpm lint