Rework context insertion UX (#12360)

- Confirming a completion now runs the command immediately
- Hitting `enter` on a line with a command now runs it
- The output of commands gets folded away and replaced with a custom
placeholder
- Eliminated ambient context

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/b1927a45-52d6-4634-acc9-2ee539c1d89a">

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

assets/keymaps/default-linux.json

@@ -211,7 +211,9 @@
       "ctrl-s": "workspace::Save",
       "ctrl->": "assistant::QuoteSelection",
       "shift-enter": "assistant::Split",
-      "ctrl-r": "assistant::CycleMessageRole"
+      "ctrl-r": "assistant::CycleMessageRole",
+      "enter": "assistant::ConfirmCommand",
+      "alt-enter": "editor::Newline"
     }
   },
   {