sidebar: Experimental Terminal Mode (#56063)

Experiment with allowing users to manage terminal sessions along with
threads in the sidebar.

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

assets/keymaps/default-linux.json

@@ -1247,6 +1247,12 @@
       "ctrl->": "agent::AddSelectionToThread",
     },
   },
+  {
+    "context": "AgentPanel && Terminal",
+    "bindings": {
+      "ctrl-n": "agent::NewThread",
+    },
+  },
   {
     "context": "ZedPredictModal",
     "bindings": {