refactor: replace find exercise job with reusable workflow

[FidelusAleksander]

Summary

The current workflow was checking for only 1 issue (the exercise issue)

 const issues = await github.rest.issues.listForRepo({
        owner: context.repo.owner,
        repo: context.repo.repo,
        state: 'open',
        per_page: 1 <- HERE
      });

Which caused an issue when one of the users tried to open a bug report but opened the issue on their copy of the exercise repository.

Changes

Use the find-exercise-issue workflow from exercise-toolkit

Closes: #18

Task list

• For workflow changes, I have verified the Actions workflows function as expected.
• For content changes, I have reviewed the style guide.

[Copilot]

Pull Request Overview

This PR refactors the “find exercise” steps from multiple workflows to use a centralized reusable workflow from exercise-toolkit.

• Replaces duplicated GitHub Script logic with a reusable workflow.
• Standardizes the “Find Exercise Issue” job across all workflows.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
.github/workflows/4b-copilot-on-github.yml	Uses reusable workflow for finding the exercise issue
.github/workflows/4-copilot-edits.yml	Uses reusable workflow for finding the exercise issue
.github/workflows/3b-copilot-agent-mode.yml	Uses reusable workflow for finding the exercise issue
.github/workflows/2-first-introduction.yml	Uses reusable workflow for finding the exercise issue
.github/workflows/1-preparing.yml	Uses reusable workflow for finding the exercise issue