Add Astro llms.txt plugin to template/blog (follow-up after Astro 5+ upgrade)

Context

In #679 we added a static template/app/public/llms.txt placeholder so users have a curated, customizable index for AI/LLM crawlers (per llmstxt.org). That covers the app/marketing side.

For the blog & docs (template/blog/), the right approach is to use an Astro integration so llms.txt (and ideally llms-small.txt / llms-full.txt) gets auto-generated from the Starlight content at build time — saving users from writing/maintaining a generator script.

Why this is a follow-up

The maintained Astro llms.txt plugins all require Astro 5+ / Starlight 0.31+:

Plugin	Astro peer dep
starlight-llms-txt (delucis)	^5.1.6 (all versions)
@4hse/astro-llms-txt	^5.1.6 || ^6.0.0
astro-llms-txt (codiume)	^5.0.0

The template blog is currently on Astro 4.16.15 / Starlight 0.29.2, so none install cleanly. The Astro 6 / Starlight 0.38 / Tailwind 4 upgrade is already in flight in #647 — this task should land after that merges.

Scope

• Pick a plugin. Recommended: starlight-llms-txt since it's Starlight-aware and generates llms.txt, llms-small.txt, and llms-full.txt from content collections automatically
• Install and configure in template/blog/astro.config.mjs
• Verify it generates expected files on npm run build
• Update the SEO guide to describe the auto-generated blog/docs llms.txt (currently only the app-side static file is documented)
• In opensaas-sh: optionally migrate from the existing custom generate-llms-txt.mjs script to the same plugin for consistency (separate decision, addressed in #679 PR review thread)

Blocked by

• #647 (Astro 6 / Starlight 0.38 / Tailwind 4 upgrade)

Related

• #679 (added static app-side llms.txt + JSON-LD schema markup)