[EPAC-2008]: Replace ci-failure-linear.yml with ci-failure-handler.yml + expand watched workflows to cover post-merge runs

[riddim-developer-bot]

Purpose

Resolves Linear issue EPAC-2008: Replace ci-failure-linear.yml with ci-failure-handler.yml + expand watched workflows to cover post-merge runs.

This adds .github/workflows/ci-failure-handler.yml that widens the workflow_run failure net to catch all critical post-merge / scheduled workflows, and removes the event == 'pull_request' filter so we catch failures on main.

Verification Evidence

• actionlint .github/workflows/ci-failure-handler.yml completed cleanly without errors.
• Checked current workflows list against YAML configurations (grep -E '^name:' .github/workflows/*.yml) and ensured names match perfectly.
• Excluded handler workflow from its own watched workflows to prevent infinite loops.
• Checked that ci-failure-linear.yml was deleted.
• Ran tests / verification steps via local workflow checks and actionlint.

Reviewer-Boundary: review-only