Fix #8: register warning stubs when no workspace folder; F5 opens the repo. Add selection context menus (#9)

- extension.ts: with no folder open, every contributed coauthoring command now
  registers a warning stub ('open a folder first') instead of being absent —
  the palette no longer errors 'command not found'. Opening a folder reloads
  the window and re-activates with a real root.
- .vscode/launch.json: the EDH now opens the repo as its workspace, so plain
  F5 lands in a working state.
- package.json: editor/context menu entries for 'Ask Claude to Edit
  Selection' and 'Add Coauthoring Thread on Selection', shown only when text
  is highlighted in a file-scheme editor (editorHasSelection).
- E2E: second no-workspace pass (suite-no-workspace) pins the #8 regression.

Fixes #8. Fixes #9.

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": ["--extensionDevelopmentPath=${workspaceFolder}"],
+      "args": ["${workspaceFolder}", "--extensionDevelopmentPath=${workspaceFolder}"],
       "outFiles": ["${workspaceFolder}/out/**/*.cjs"],
       "preLaunchTask": "npm: build"
     }