docs(readme): link the built-in agent bullet to its product page

[leochenpm]

What

One link, in the Relationship to Orkas section.

-- **Inside Orkas** it ships as a built-in agent — no separate install; the zero-key trunk
-  works out of the box.
+- **Inside Orkas** it ships as a [built-in agent](https://orkas.ai/agents/video-studio/?source=gh-orkas-vs) —
+  no separate install; the zero-key trunk works out of the box.

Why this bullet, and why nothing else changed

The section already had two orkas.ai links and both were left alone — they
point at the homepage, which is right where they sit: the opening sentence is
about the product this toolkit was extracted from, and the Links: line is
navigation.

The Inside Orkas bullet had no link at all, even though it is the one place in
the README that describes the built-in agent specifically. /agents/video-studio/
is the page documenting exactly that, so a reader following it lands on what the
sentence promised instead of on the homepage.

The anchor text is the wording that was already there. No copy was reworded to
carry a link.

Verified

• https://orkas.ai/agents/video-studio/?source=gh-orkas-vs → 200, title
  VideoStudio — Create and intelligently edit video in Orkas. No redirect, no 404.
• README.md is listed under targetOwned in the upstream sync-rules.json, so
  this file is maintained by this repository and the Orkas-side sync will not
  overwrite it.
• Docs-only change; it cannot affect typecheck or the test suites, so CI is the
  check that matters here.

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