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>

README.md

@@ -26,6 +26,9 @@ human/Claude attribution (Feature #6).
 1. `npm install`
 2. `npm run build`
 3. Press **F5** (or Run → "Run Extension") to launch the Extension Development Host.
+   It opens the committed **`sandbox/`** playground as its workspace (not the
+   repo itself — VS Code won't open one folder in two windows, #8); start with
+   `sandbox/playground.md`.
 4. In the new window: **Cmd/Ctrl+Shift+P** → **"Cowriting: Show Cline SDK Info"**.
 
 ## F2 — Region-anchored threads (Feature #4)