chore: configure markdownlint VS Code extension

Install and configure markdownlint extension for in-editor linting with
visual feedback (squiggly lines, Problems panel, quick fixes).

Changes:
- Install davidanson.vscode-markdownlint extension
- Configure .vscode/settings.json to use .markdownlint.json
- Enable immediate visual feedback for markdown style issues

Benefits:
- Real-time markdown linting while editing
- Quick fixes via lightbulb (Ctrl+.)
- Problems panel integration (Ctrl+Shift+M)
- Consistent with CLI linting configuration

.vscode/settings.json

@@ -18,5 +18,9 @@
         "titleBar.activeForeground": "#15202b",
         "titleBar.inactiveBackground": "#61dafb99",
         "titleBar.inactiveForeground": "#15202b99"
+    },
+    // Markdown linting
+    "markdownlint.config": {
+        "extends": ".markdownlint.json"
     }
 }