fix(desktop): project New task action does not exit WorkHub

## What happened

When WorkHub is enabled and active in Maka Desktop, choosing **New task** from a Project row’s three-dot menu does not leave WorkHub. The underlying new-task state is selected, but the WorkHub dock stays visible, so the action appears to do nothing and the user cannot interact with the new-task composer.

Expected: this action should behave like the sidebar’s top-level **New task** action: exit WorkHub, open a blank task scoped to the chosen Project, and focus the composer.

## How to reproduce

1. Open Maka Desktop with at least one Project and one existing task.
2. Enable WorkHub and leave the WorkHub dock active.
3. In the task sidebar, switch grouping to **By project**.
4. Open a Project row’s three-dot menu.
5. Choose **New task**.
6. Observe that the WorkHub dock remains visible and the new-task composer is not presented.

An Electron Playwright regression reproduces this deterministically by asserting that `.workHubDock` becomes hidden after the menu action. On current `main`, it remains visible for the full 10-second assertion timeout.

## Environment

- Maka commit: `c60e12cb`
- OS: Ubuntu Linux (headless Electron via Xvfb)
- Surface: Desktop / WorkHub
- Node.js: `v24.18.1`

## Additional context

`SessionNavigationProvider` wraps the top-level `onNew` callback with `onExitWorkHub()`, but its memoized `projectActions.onNew` forwards directly to `AppShell` without exiting WorkHub. `createSessionInProject()` opens the new-task state but also does not exit WorkHub.