Pipeline Resilience Phase 2: Add failure alerts to silent workflows

[madjin]

Context

4 of 11 GitHub Actions workflows have zero failure alerting. When they fail, the only way to discover it is manually checking the Actions tab. The other 7 workflows already use .github/actions/alert-failure to post to Discord.

Scope

Add the alert-failure action to the end of each workflow's job steps:

- name: Alert on failure
  if: failure()
  uses: ./.github/actions/alert-failure
  with:
    webhook-url: ${{ secrets.ALERT_WEBHOOK_URL }}
    workflow-name: '<Workflow Name>'

Files to modify

File	Workflow name
.github/workflows/update_hackmd_notes.yml	Update HackMD Notes
.github/workflows/generate-posters.yml	Generate Content Posters
.github/workflows/daily_discord_briefing.yml	Daily Discord Facts Briefing
.github/workflows/knowledge-gh-pages.yml	Deploy to GitHub Pages

~20 lines total across 4 files.

Verification

Validate all workflow YAMLs parse correctly after changes.

References

• Existing usage in extract_daily_facts.yml, sync.yml, retro.yml etc.