Streamline Pages workflows and add PR build guard

[Codex]

Automation asked for: identify/correct deploy and jekyll workflows, improve deployment automation, and optimize CI behavior.

• Deploy workflow: Build on PRs for early breakage detection; keep Pages config/artifact/upload/deploy only on push; trim permissions per job; drop node_modules before artifact upload to avoid bloat.
• Build check (jekyll.yml): Replace placeholder with a PR/scheduled Node 20 build to validate asset generation without deploying.

Example (deploy guardrails):

on:
  push:
    branches: [gh-pages]
  pull_request:
    branches: [gh-pages]

jobs:
  build:
    runs-on: ubuntu-latest
    # Pages steps only on push
    - name: Setup Pages
      if: github.event_name == 'push'
      uses: actions/configure-pages@v4

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Automatiser et corriger les workflows de déploiement</issue_title>
<issue_description>Suite à la discussion sur l'automatisation de mon dépôt GitHub, je souhaite que vous inspectiez mon dépôt DavidKRK.github.io et les workflows existants pour:

• Identifier les erreurs ou problèmes possibles dans les workflows déploy.yml et jekyll.yml.
• Proposer des améliorations pour automatiser les tâches de déploiement et de maintenance.
• Corriger les erreurs et optimiser les performances des workflows.

Veuillez créer un pull request avec les modifications nécessaires pour chaque dépôt que vous souhaitez inspecter.</issue_description>

Comments on the Issue (you are @codex[agent] in this section)

• Fixes Automatiser et corriger les workflows de déploiement #125

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