build: leave comment when slash command fails

---
type: pre_commit_static_analysis_report
description: Results of running static analysis checks when committing changes.
report:
  - task: lint_filenames
    status: passed
  - task: lint_editorconfig
    status: passed
  - task: lint_markdown
    status: na
  - task: lint_package_json
    status: na
  - task: lint_repl_help
    status: na
  - task: lint_javascript_src
    status: na
  - task: lint_javascript_cli
    status: na
  - task: lint_javascript_examples
    status: na
  - task: lint_javascript_tests
    status: na
  - task: lint_javascript_benchmarks
    status: na
  - task: lint_python
    status: na
  - task: lint_r
    status: na
  - task: lint_c_src
    status: na
  - task: lint_c_examples
    status: na
  - task: lint_c_benchmarks
    status: na
  - task: lint_c_tests_fixtures
    status: na
  - task: lint_shell
    status: na
  - task: lint_typescript_declarations
    status: na
  - task: lint_typescript_tests
    status: na
  - task: lint_license_headers
    status: passed
---

Author: Planeshifter

.github/workflows/slash_commands.yml

@@ -246,6 +246,25 @@ jobs:
 
     # Define the job's steps:
     steps:
+      # Check if any previous job failed and post a comment:
+      - name: 'Notify of command failure'
+        if: |
+          contains(needs.*.result, 'failure')
+        uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
+        with:
+          github-token: ${{ secrets.STDLIB_BOT_PAT_REPO_WRITE }}
+          script: |
+            const commentBody = context.payload.comment.body.trim();
+            const lines = commentBody.split( '\n' );
+            const quote = lines.map( line => `> ${line}` ).join( '\n' );
+
+            await github.rest.issues.createComment({
+              'owner': context.repo.owner,
+              'repo': context.repo.repo,
+              'issue_number': context.issue.number,
+              'body': `${quote}\n\n@${context.payload.comment.user.login}, the slash command failed to complete. Please check the workflow logs for details.\n\n[View workflow run](https://github.com/${context.repo.owner}/${context.repo.repo}/actions/runs/${context.runId})`
+            });
+
       - name: Remove in-progress label
         # Run the step regardless of the outcome of previous steps:
         if: always()