docs: add karesansui hero image from CDN (#115)

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Model-Raw: claude-opus-4-5-20251101

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: nullvariant

extensions/git-id-switcher/docs/DESIGN_PHILOSOPHY.md

@@ -1,5 +1,9 @@
 # Karesansui Architecture
 
+<p align="center">
+  <img src="https://assets.nullvariant.com/nullvariant-vscode-extensions/extensions/git-id-switcher/images/karesansui.webp" alt="Digital Karesansui (デジタル枯山水)" width="600">
+</p>
+
 *A design philosophy for Git ID Switcher*
 
 > **Note**: Karesansui (枯山水) is a Japanese rock garden--sand and stone representing mountains and water.