Internal page links bypass Next.js client navigation

[adliebe]

Several internal page links still use raw <a href="/..."> anchors instead of Next.js Link.

Repro:

1. Run pnpm lint on current master.
2. Observe @next/next/no-html-link-for-pages warnings on internal routes, including:
   • src/app/blog/[slug]/page.tsx linking to /blog
   • src/app/for-hire/[[...tags]]/page.tsx linking to /gigs
   • src/app/gigs/[[...tags]]/page.tsx linking to /for-hire
   • src/app/investors/page.tsx linking to /funding and /mcp

Why it matters:
Raw internal anchors force full document navigation instead of Next.js client-side navigation, which is slower and loses app state unnecessarily.

Expected:
Internal app routes should use next/link; external/mailto links should remain anchors.