Agent clicks on _blank links in the Browser panel can steal composer focus #29009

When an agent clicks a target="_blank" link in a desktop-hosted Browser tab, the composer can lose keyboard focus. This also happens when the agent works in another thread.

Path: apps/desktop/src/preview/Manager.ts window open handler (foreground-tab) → emitOpenLink → web openLinkInPreview (apps/web/src/browser/ElectronBrowserHost.tsx) → openUrlInPreview → openBrowser. openBrowser is a user action, so startWorkspaceFocus (apps/web/src/lib/workspaceFocus.ts) requests panel focus.

The desktop does not tell the web whether the click came from the agent or the user. Two options:

• Open the tab in the background when the click came from the agent.
• Make startWorkspaceFocus request panel focus only for the thread on screen. This closes the cross-thread part.

Found during review of #28613.