agent: Add new panel navigation dropdown (#29539)

- [x] Ensure what appears in the dropdown is really what is accurate
- [x] Ensure keyboard navigation works:
  - [x] Switching tabs with `enter`
  - [x] Closing items from the menu item
  - [x] Opening the dropdown
  - [x] Focus assistant panel on dismiss
- [x] Add ability to close items from the dropdown menu
- [x] Persistence
- [x] Correct behavior when opening a text thread

Release Notes:

- agent: Added a navigation menu that shows the recently opened threads.
The button to see the full history view has been changed inside this
menu.

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Cole Miller <m@cole-miller.net>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Cole Miller <cole@zed.dev>

assets/keymaps/default-linux.json

@@ -250,6 +250,12 @@
       "ctrl-alt-e": "agent::RemoveAllContext"
     }
   },
+  {
+    "context": "AgentPanel > NavigationMenu",
+    "bindings": {
+      "ctrl-backspace": "agent::DeleteRecentlyOpenThread"
+    }
+  },
   {
     "context": "AgentPanel > Markdown",
     "bindings": {