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>

sandbox/playground.md

@@ -0,0 +1,21 @@
+# Cowriting sandbox
+
+This folder is the Extension Development Host's workspace (see
+`.vscode/launch.json`): F5 opens it instead of the extension repo itself,
+because VS Code won't open one folder in two windows and the repo is already
+open in your dev window (#8).
+
+Play here:
+
+1. Type a sentence below — it renders with the human left-border as you type.
+2. Highlight it → right-click → **Ask Claude to Edit Selection** — the
+   replacement lands Claude-tinted.
+3. Edit inside the tinted span — it splits character-precisely.
+4. Save, then peek at `sandbox/.threads/playground.md.json` — the git-native
+   attribution record. Reopen the file: spans re-resolve.
+5. Highlight text → right-click → **Add Coauthoring Thread on Selection** for
+   an anchored discussion thread.
+
+---
+
+The quick brown fox jumps over the lazy dog.