UI improvements And Ai features

## Summary

A set of related titlebar/agent-window UI suggestions inspired by how Devin Desktop handles its agent panel. Right now the agent window in VS Code opens as a separate window, which breaks flow and makes the UI feel cluttered compared to Devin Desktop's approach. This request bundles several changes that together would make the titlebar cleaner and the agent workflow faster.

## 1. Turn the agent window button into a titlebar slider

Instead of a button like Devin Desktop has in the top-right corner that opens a whole new window, add an agents window button directly in the titlebar and make it a slider/toggle.

Instead of needing to open a new window for this, you'd just switch with the slider — plus a keyboard shortcut to toggle it. Bonus: a cleaner titlebar style like this would honestly make VS Code look better overall.

## 2. Bring @ handle context support to the agent window

Not everything here is UI — some of it's AI functionality. In Devin Desktop you can use `@` to bring in all sorts of things as context. In VS Code's agent window, you can't.

Adding this would take VS Code's agent experience to the next level.

## 3. Send all problems directly to the agent

Right now there's no quick way to send every problem in the Problems panel straight to the agent in one action.

## 4. Cleaner titlebar layout

Another UI change that would make VS Code look cleaner overall:

(Ignore the Devin Desktop auth icon in that image — not asking for that, unless the team wants to consider adding Microsoft account sign-in there too.)

## 5. Move sidebar icons into titlebar buttons

Remove these:

And re-add them as buttons in the titlebar instead. Would make the overall UI look a lot neater.

## Why this matters

Together these changes would cut down on window-switching, bring VS Code's agent context handling closer to parity with tools like Devin Desktop, and give the whole editor a cleaner, more modern titlebar.