feat(astro): add --astro flag and wire the Astro starter into the pipeline

[PAMulligan]

Summary

Completes the v2.0.0 multi-framework output (Next.js, Vite, Astro) goal by making Astro a fully wired, first-class output target (#78).

templates/astro/ already exists as a complete hybrid-islands starter (Astro 5 + React islands + Tailwind); this PR wires it into the scaffolder and the pipeline config, documents it, and verifies it builds end-to-end.

Changes

• scripts/setup-project.sh — adds the --astro flag (alias for --renderer astro) and updated usage. The Astro path now copies templates/astro/ (deterministic, offline) rather than scaffolding via pnpm create astro, since we ship a complete starter; redundant dep installs are skipped.
• .claude/pipeline.config.json — recognizes Astro inputs (astro.config.* in the config cache category, src/**/*.astro in source). Validated against the schema.
• docs/multi-framework/README.md — documents that Astro maps to the web-app app type (app type describes the kind of app — web-app / chrome-extension / pwa / react-native — while the framework is the orthogonal renderer).
• README.md / templates/README.md — list Astro alongside Next.js and Vite (quickstart, template tree, template table).
• scripts/__tests__/setup-project.test.js — adds a --astro --dry-run alias test.

Acceptance criteria

• templates/astro/ exists with a minimal working Astro starter (Astro 5, React islands, Tailwind) — already present, now verified buildable
• scripts/setup-project.sh accepts an --astro flag
• Pipeline config knows about Astro (config/source patterns) and it's documented that Astro falls under the web-app app type
• README updated
• Smoke test passes

Smoke test

Verified locally end-to-end:

$ ./scripts/setup-project.sh demo-astro --astro && cd demo-astro && pnpm install && pnpm build
…
[build] 1 page(s) built in 2.00s
[build] Complete!
$ ls dist/   # _astro/  index.html

(The issue's cd app is the repo's documentation convention — the project is created at the given name, e.g. demo-astro, exactly as the README's own quickstart shows.)

Testing

• setup-project.test.js (7 tests incl. the new --astro alias) ✅
• scripts/validate-pipeline-config.js ✅ (schema-valid), check-doc-counts ✅
• Repo-wide prettier --check . ✅, eslint . (0 errors)
• Full setup-project.sh demo-astro --astro → pnpm install → pnpm build produces dist/ ✅

Refs #78