fix(ci): use underscore input names for first-interaction@v3 (#22)

v3 renamed inputs to repo_token / issue_message / pr_message (underscores).
With v1 hyphenated names, the action errors:
  "Input required and not supplied: issue_message"
on every PR. Mirrors opendecree/decree#188.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: zeevdr

.github/workflows/welcome.yml

@@ -17,12 +17,12 @@ jobs:
     steps:
       - uses: actions/first-interaction@v3
         with:
-          repo-token: ${{ secrets.GITHUB_TOKEN }}
-          issue-message: |
+          repo_token: ${{ secrets.GITHUB_TOKEN }}
+          issue_message: |
             Thanks for opening your first issue! We'll take a look soon.
 
             In the meantime, search [Discussions](https://github.com/opendecree/decree/discussions) for related topics.
-          pr-message: |
+          pr_message: |
             Thanks for your first pull request! We appreciate the contribution.
 
             Before review, please make sure tests pass: `cd sdk && pytest`