Add Astro template (templates/astro/)

[PAMulligan]

Goal

The v2.0.0 milestone explicitly calls out "multi-framework output (Next.js, Vite, Astro)". Next.js and Vite already exist under templates/; Astro does not.

Acceptance criteria

• templates/astro/ directory created with a minimal working Astro starter
• scripts/setup-project.sh accepts an --astro flag
• Pipeline config knows about Astro app type (or documented why it falls under "web app")
• README updated
• Smoke test: ./scripts/setup-project.sh demo-astro --astro && cd app && pnpm install && pnpm build succeeds

Milestone

v2.0.0