Agent Skills: astra-ssg

Use when building markdown-driven docs / blog sites with `mizchi/astra`. Astra is a static-site generator first — `astra build` writes a pure-static tree to disk and `astra dev` runs a local preview server. The Mars-middleware form (`mizchi/astra/middleware`) is a secondary shape for when you need to mount the same content on a long-running server (e.g. sol). `astra build` itself does not open a network listener and (as of 0.22.3) no longer instantiates a Mars `Server` — `build_to_disk` calls `Middleware::render_url(url)` directly — so the typical deploy is "build once, serve from any static host" (CF Workers Static Assets, GitHub Pages, S3, etc.). Covers (a) the file-to-URL routing rules under `docs/` (every `.md` becomes a route; `index.md` becomes the directory root; `ja/` and other locale dirs are i18n branches), (b) `astra.config.json` fields (`docs_dir`, `out_dir`, `nav`, `sidebar`, `i18n`, `islands`, `spaRoutes`, `deploy`, `headSnippets`), (c) per-page metadata via frontmatter (`title`, `description`, `layout`, `sidebar`, `revalidate` for ISR — the last one only matters when served from Mars) and `page.json` (`ssr`, `renderer`, `spa`, `fallbackBehavior`, `staticParams`), (d) three ways to drop components onto a page — TSX with React `renderToString` SSR, MoonBit `.mbt` with `pub fn render() -> Node[Unit]`, or client-side web components in the islands directory, (e) when to reach for the middleware mount instead (sol embedding, on-demand revalidation), and (f) how this composes with sol (no edge: astra has no sol dependency).