feat: upgrade Node.js runtime from node20 to node24 (#123)

GitHub Actions is deprecating the Node.js 20 runtime, with enforcement
starting June 2, 2026. This updates the action to use node24 runtime.

- action.yml: change `using: "node20"` to `using: "node24"`
- package.json: bump `@types/node` from ^20.16.5 to ^24

Fixes #122

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -12,7 +12,7 @@
     "@actions/tool-cache": "^2.0.2",
     "semver": "^7.7.1",
     "undici": "^7.8.0",
-    "@types/node": "^20.16.5",
+    "@types/node": "^24",
     "@types/semver": "^7.7.0",
     "tsdown": "0.10.1"
   }