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>

test/e2e/runTest.ts

@@ -20,6 +20,14 @@ async function main(): Promise<void> {
       launchArgs: [workspace, "--disable-extensions"],
       extensionTestsEnv: { E2E_WORKSPACE: workspace },
     });
+
+    // Second pass — NO workspace folder (regression for #8): commands must
+    // exist as warning stubs instead of erroring "command not found".
+    await runTests({
+      extensionDevelopmentPath,
+      extensionTestsPath: path.resolve(__dirname, "./suite-no-workspace/index"),
+      launchArgs: ["--disable-extensions"],
+    });
   } finally {
     fs.rmSync(workspace, { recursive: true, force: true });
   }