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>
(cherry picked from commit 6a293fc1a2)

action.yml

@@ -26,7 +26,7 @@ outputs:
   release-channel:
     description: "The release channel of the installed version."
 runs:
-  using: "node20"
+  using: "node24"
   main: "dist/main.mjs"
   post: "dist/post.mjs"
   post-if: always()