fix: update Node.js runtime to v24 LTS (#30)

Update the Node.js runtime from `node20` to `node24` (latest LTS).

## Changes
- `action.yml`: `runs.using` updated to `node24`
- `package.json`: `engines.node` updated to `>=24.0.0`

Node.js 20 reaches end-of-life on April 30, 2026. Node.js 24 became LTS
in October 2025 and is supported until April 2028.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ueokande

action.yml

@@ -8,5 +8,5 @@ inputs:
     description: 'The GitHub token to use for requests to the GitHub API. Allows a higher rate limit.'
 
 runs:
-  using: 'node20'
+  using: 'node24'
   main: 'index.js'