feat: serve each page's markdown at its own URL plus .md

[abernier]

Stacked on #639.

The markdown of a page moves from /md/<path>.md to the page URL plus .md, e.g. /getting-started/introduction.md. The "Copy Page" and "View as Markdown" actions use the new URL.

Why /md existed

The markdown is served by a route handler, and a route handler cannot share a segment with [...slug]/page.tsx (nor sit in a sibling catch-all with another slug name). #615 therefore gave it its own /md prefix. The handler stays where it is, src/app/md/[...slug]/route.ts (with dynamicParams = false from #639), but /md is now an internal path, exposed differently in each mode:

Server mode (Vercel, next start)

• A rewrite in next.config.mjs maps /:path+.md to /md/:path+.md. A default (afterFiles) rewrite is checked after public files and static pages but before dynamic routes, so the [...slug] catch-all never sees these paths. basePath is prefixed onto both sides by Next. :path+ rather than :path*, so /.md is not rewritten to /md.md.
• The rewrite is only set when not exporting: with output: 'export' Next would warn about it.
• Old URLs: a permanent redirect from /md/:path+.md to /:path+.md. Redirects only apply to the incoming request, never to a rewrite's destination, so it cannot loop with the rewrite (checked: one 308, then 200).
• Unknown /<path>.md is a 404, with MDX unset at runtime: the rewritten path hits dynamicParams = false.

Static export (OUTPUT=export, CLI --format website, GitHub Pages)

There are no rewrites in a static export, so the files are moved instead: the export writes md/<path>.md, and both export entry points move them beside the pages, at <path>.md, leaving no md/ behind:

• next-build.sh (pnpm run build, used by start.sh and Playwright), in ${DIST_DIR:-out};
• src/cli/website.ts (npx @pmndrs/docs build ... --format website, used by the reusable build.yml), when copying the export to the out dir.

The base path is not part of the exported file layout, so it needs no handling there. Old /md/<path>.md URLs are a 404 on a static export.

Verification (local)

• pnpm run lint, pnpm exec tsc --noEmit, pnpm exec vitest run (390 tests): pass.
• Server build, then next start with MDX unset:
  • /getting-started/introduction.md: 200 text/markdown; charset=utf-8, the page's markdown
  • /getting-started/nope.md, /nope.md, /md.md: 404
  • /md/getting-started/introduction.md: 308 to /getting-started/introduction.md, then 200 text/markdown
  • /getting-started/introduction: 200, its HTML carries /getting-started/introduction.md
  • POST /api/mcp tools/list: 200
• Static export via next-build.sh: 20 .md files beside the pages, no out/md; same with DIST_DIR=build-distdir BASE_PATH=/sub (links read /sub/getting-started/introduction.md). The CLI's --format website with BASE_PATH=/sub: same. Served with serve: /getting-started/introduction.md 200 text/markdown, /md/... 404.
• npx playwright test --update-snapshots against start.sh: 74 passed.

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