chore: enable WakaTime in devcontainer (#2)

Pass WAKATIME_API_KEY through remoteEnv and install the WakaTime
VS Code extension so coding activity is tracked inside the container.
Extension lists are sorted alphabetically.

Co-authored-by: tkgstrator <tkgstrator@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

.devcontainer/devcontainer.json

@@ -65,6 +65,7 @@
       "extensions": [
         "EditorConfig.EditorConfig",
         "PKief.material-icon-theme",
+        "WakaTime.vscode-wakatime",
         "antfu.file-nesting",
         "bierner.markdown-mermaid",
         "bierner.markdown-preview-github-styles",
@@ -78,4 +79,4 @@
       "settings": {}
     }
   }
-}
+}