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>

package.json

@@ -36,6 +36,18 @@
       "commandPalette": [
         { "command": "cowriting.applyAgentEdit", "when": "false" }
       ],
+      "editor/context": [
+        {
+          "command": "cowriting.editSelection",
+          "when": "editorHasSelection && resourceScheme == file",
+          "group": "1_cowriting@1"
+        },
+        {
+          "command": "cowriting.createThread",
+          "when": "editorHasSelection && resourceScheme == file",
+          "group": "1_cowriting@2"
+        }
+      ],
       "comments/commentThread/context": [
         { "command": "cowriting.reply", "group": "inline", "when": "commentController == cowriting.threads" }
       ],