ci(blog): deploy Astro build to GitHub Pages

[alabulei1]

Why

GitHub Pages was configured as a legacy build serving raw files from the main branch root, so the Astro output in dist/ was never published. Consequences:

• Markdown-only articles (e.g. gpt-5-6-token-station) returned 404 — no committed blog/<slug>.html existed.
• The listing read the stale root posts.json, so new articles never appeared.
• The Astro migration (PRs build(blog): introduce Astro framework for writings #13/Refactor existing blog article URLs with Astro #14/docs(blog): add GPT-5.6 Token Station article #15) was effectively not wired to production.

What

• Add a deploy job to blog-framework.yml that builds dist/ and publishes it with actions/upload-pages-artifact + actions/deploy-pages, on every push to main, gated behind build-and-check.
• Add pages: write / id-token: write permissions, the github-pages environment, and a pages concurrency group.
• Push trigger no longer path-filtered, so root chrome (index*.html, sitemap.xml, CNAME) redeploys too.
• Repo Pages source switched to "GitHub Actions" (done via API) to match.

After merge

The push to main runs build + deploy; /posts.json and /blog/gpt-5-6-token-station.html (all four languages) go live. Future articles need only their Markdown.

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