Fix Welcome New Contributors workflow: replace broken first-interaction action

[Copilot]

actions/first-interaction has a known bug that causes it to always post the welcome comment, regardless of whether the author is a returning contributor.

Changes

• Removes actions/first-interaction entirely
• Adds a Check if first PR step using actions/github-script that queries the GitHub Search API for the author's prior PRs in the repo — returns true only when total_count <= 1
• Adds a Welcome New Contributor step that posts the comment only when the author is not a bot and the first-PR check passes

- name: Check if first PR
  id: first-pr-check
  if: steps.bot-author-check.outputs.result != 'true'
  uses: actions/github-script@... # v8
  with:
    result-encoding: string
    script: |
      const { data } = await github.rest.search.issuesAndPullRequests({
        q: `is:pr repo:${context.repo.owner}/${context.repo.repo} author:${context.payload.sender.login}`,
        per_page: 2,
      });
      return String(data.total_count <= 1);

- name: Welcome New Contributor
  if: steps.bot-author-check.outputs.result != 'true' && steps.first-pr-check.outputs.result == 'true'
  ...

The per_page: 2 cap keeps the search query cheap; total_count <= 1 handles the case where the search index hasn't yet indexed the current PR.

[Copilot]

Pull request overview

Updates the reusable “Welcome New Contributors” GitHub Actions workflow to avoid the actions/first-interaction bug that posts welcome comments for returning contributors, by replacing it with an explicit first-PR check and a conditional comment step.

Changes:

• Remove actions/first-interaction usage.
• Add a GitHub Search API check (via actions/github-script) to determine whether the PR author has prior PRs in the repo.
• Post the welcome comment only when the author is not a bot and the first-PR check passes.

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