Agents: Explore top-level menu for Windows/Linux

[bamurtaugh]

Top-level menu items available in VS Code (like File, Edit, etc) are not available on Windows/Linux for the Agents app.

I had thought this was an intentional choice for UI simplification, though I have been missing some of these options (I find myself wishing I could reach for them in menu UX rather than having to go to the command palette), and since I believe they're available on macOS, we may want to discuss a plan for Windows/Linux.

[bamurtaugh]

From @osortega:

I'm also not sure what we need from that menu specifically beyond the obvious things for navigation, we might want to prune that menu before adding it

From @meganrogge:

menus are heavily used by screen reader users. We need a new place to show the Screen Reader Optimized status bar indicator if we don't have that in this window too.