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>

.gitignore

@@ -5,3 +5,6 @@ out/
 
 # E2E throwaway sidecars (the harness copies the fixture to a tmpdir, but guard anyway)
 test/e2e/fixtures/workspace/.threads/
+
+# Sandbox playground churn (the EDH workspace - play freely, commit nothing)
+sandbox/.threads/

.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"
     }

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)

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.