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/suite-no-workspace/index.ts

@@ -0,0 +1,13 @@
+import * as path from "path";
+import Mocha from "mocha";
+import { glob } from "glob";
+
+export async function run(): Promise<void> {
+  const mocha = new Mocha({ ui: "tdd", color: true, timeout: 60000 });
+  const testsRoot = path.resolve(__dirname);
+  const files = await glob("**/*.test.js", { cwd: testsRoot });
+  for (const f of files) mocha.addFile(path.resolve(testsRoot, f));
+  await new Promise<void>((resolve, reject) => {
+    mocha.run((failures) => (failures > 0 ? reject(new Error(`${failures} E2E test(s) failed`)) : resolve()));
+  });
+}