chore: migrate example directory from yarn v1 to pnpm

- Add packageManager field to example/package.json
- Convert example/yarn.lock to example/pnpm-lock.yaml
- Remove example/yarn.lock

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: azu

example/package.json

@@ -18,5 +18,6 @@
   },
   "dependencies": {
     "textlint-rule-preset-jtf-style": "file:.."
-  }
+  },
+  "packageManager": "[email protected]+sha512.9fb969fa749b3ade6035e0f109f0b8a60b5d08a1a87fdf72e337da90dcc93336e2280ca4e44f2358a649b83c17959e9993e777c2080879f3801e6f0d999ad3dd"
 }