Framework adapter: Astro

Add auto-detection and snippet patching for Astro projects.

Acceptance criteria

Detect Astro by astro.config.* at the project root
Insert the snippet into src/layouts/Layout.astro (or the file matching src/layouts/*.astro)
Use the `is:inline` attribute on the script tag so Astro's bundler doesn't process it
Preserve existing Astro frontmatter and styling
Update `docs/install-recipes.md` framework matrix