fix: show a focus outline on the copy button

`.copy-button` set `outline: none` unconditionally, so the button was
reachable by keyboard but gave no visible indication of focus. Drop it and
draw an explicit ring on `:focus-visible`, using `--color-text-primary` so it
contrasts in both light and dark mode.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01THEmqTTNhxu1QbR7isfQ5M

Author: OmerDavidson

.changeset/copy-button-focus.md

@@ -0,0 +1,5 @@
+---
+'@node-core/doc-kit-legacy': patch
+---
+
+Show a focus outline on the code block copy button

packages/node-legacy/src/legacy-html/assets/style.css

@@ -1032,7 +1032,6 @@ kbd {
 }
 
 .copy-button {
-  outline: none;
   font-size: 10px;
   color: #fff;
   background-color: var(--green1);
@@ -1055,6 +1054,11 @@ kbd {
   background-color: var(--green2);
 }
 
+.copy-button:focus-visible {
+  outline: 2px solid var(--color-text-primary);
+  outline-offset: 2px;
+}
+
 :root:not(.has-js) .copy-button {
   display: none;
 }