Fix #8 (follow-up): point the EDH at a committed sandbox/ playground

${workspaceFolder} as the EDH workspace collides with the dev window (VS
Code one-folder-one-window), so plain F5 still landed folderless — now with
the stub warning instead of 'command not found'. The EDH opens
sandbox/ (committed playground with a guided playground.md) instead; its
.threads/ churn is gitignored; README updated.

Fixes #8.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.vscode/launch.json

@@ -5,7 +5,7 @@
       "name": "Run Extension",
       "type": "extensionHost",
       "request": "launch",
-      "args": ["${workspaceFolder}", "--extensionDevelopmentPath=${workspaceFolder}"],
+      "args": ["${workspaceFolder}/sandbox", "--extensionDevelopmentPath=${workspaceFolder}"],
       "outFiles": ["${workspaceFolder}/out/**/*.cjs"],
       "preLaunchTask": "npm: build"
     }