Add sitemap generation and bunext sitemap command

- Add generateSitemap util (exported as bunext.bunextGenerateSitemap)
  that builds sitemap.xml from static pages, with exclude patterns,
  additional paths (array or sync/async function), lastmod from file
  mtime, trailing slash, and default changefreq/priority options
- Let page .server.ts files export a `sitemap` function to list URLs
  for dynamic routes, or override a static page's entry
- Add `bunext sitemap` command reading `sitemap` from bunext.config.ts,
  writing public/sitemap.xml by default, with --output/--origin flags
- Fix handle-routes test leaking a grab-router module mock into
  bunext-req-handler tests; set global.BUNEXT_ROUTER instead
- Ignore /.tmp
- Document sitemap usage in README and CLAUDE.md

Co-Authored-By: Claude Opus 5.5 <[email protected]>

CLAUDE.md

@@ -19,6 +19,7 @@ bun run dev         # tsc --watch
 bunext dev          # Start dev server with HMR and file watcher
 bunext build        # Bundle all pages for production
 bunext start        # Start production server from pre-built artifacts
+bunext sitemap      # Write public/sitemap.xml using config.sitemap
 ```
 
 ## Architecture
@@ -51,6 +52,7 @@ Pages live in `src/pages/`. Server logic is **separated from page files** into c
 
 **Server file** (`page.server.ts` or `page.server.tsx`) — server-only, never sent to the browser:
 - Default export or `export const server`: `BunextPageServerFn` — runs server-side before rendering, return value becomes props
+- `export const sitemap`: `BunextPageSitemapFn` — optional; returns the page's sitemap entries (how dynamic routes get into `bunext sitemap`)
 
 The framework resolves the companion by replacing the page extension with `.server.ts` or `.server.tsx`. If neither exists, no server function runs and only the default `url` prop is injected.