Add shellcheck GitHub Action with annotations for .sh files

[Copilot]

Adds automated shellcheck linting on PRs that modify shell scripts, surfacing errors as GitHub Checks annotations.

Changes

• New workflow: .github/workflows/shellcheck.yml
  • Triggers on .sh file changes in PRs to main
  • Uses reviewdog/action-shellcheck@v1 with github-check reporter for inline annotations
  • Scans all shell scripts with info-level reporting
  • Includes concurrency control to cancel stale runs

Implementation Details

• Permissions: checks: write added for annotation support
• No conflicts: Verified shellcheck not used elsewhere in workflows or pipelines (27 shell scripts across cli/installer, extensions, eng/scripts)
• Pattern matching: "*.sh" covers all shell scripts recursively

- name: Run ShellCheck
  uses: reviewdog/action-shellcheck@v1
  with:
    github_token: ${{ secrets.GITHUB_TOKEN }}
    reporter: github-check
    path: "."
    pattern: "*.sh"
    level: info

Original prompt

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This section details on the original issue you should resolve

<issue_title>[EngSys] Add GitHub Action that runs shellcheck on PRs that change .sh files</issue_title>
<issue_description>Surface errors in file tab using GitHub Actions annotations</issue_description>

<agent_instructions>Make sure that shellcheck isn't used anywhere else in DevOps pipelines or workflows. If it is we'll have to rethink.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [EngSys] Add GitHub Action that runs shellcheck on PRs that change .sh files #6621

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