docs(reflow): strip .md from internal links so hub routes resolve
The hub's /reflow/* route appends .md to the slug when fetching from
the docs repo (pages/reflow.tsx: filename = \`\${slug}.md\`), so a link
ending in .md produced a 404 (e.g. /reflow/how-to/interpret-the-output.md
fetched interpret-the-output.md.md).
Rewrites every relative markdown link to drop the .md extension while
leaving absolute https:// links (GitHub, external repos) alone. Anchors
like foo.md#bar become foo#bar.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>