ci: add PR requirements check workflow

[oto-macenauer]

Summary

Add a GitHub Actions workflow that validates pull request requirements automatically on every PR.

Motivation

PRs should meet baseline quality standards (consistent title format, a real description, a linked issue) before review. Enforcing this in CI keeps the bar consistent without manual reminders.

Proposed Solution

Add .github/workflows/check-pr-requirements.yml using the AbsaOSS/check-pr-requirements action.

• Pin the action to a commit SHA (supply-chain hardening), with a comment noting the version.
• Trigger on pull_request events: opened, synchronize, reopened, edited, labeled, unlabeled.
• Enable checks: title format, description presence, issue reference.

Acceptance Criteria

• Workflow runs on PR open/update.
• Action referenced by commit SHA, not a mutable tag.
• Title, description, and issue-reference checks enabled.