agent: Refine naming for the panel default_view setting (#31446)

Follow up to https://github.com/zed-industries/zed/pull/31353. Just
ensuring we're walking toward a more consistent use of the multiple
terms we have floating around in the AI realm. In this case, `thread` is
the term for the now default view, the one that has agentic features;
`text_thread` is the term for the original view, the one where it's just
text. The settings now reflect this. Also took advantage of the
opportunity to add some docs, too.

Release Notes:

- N/A

docs/src/ai/configuration.md

@@ -460,7 +460,7 @@ However, you can change it either via the model dropdown in the Agent Panel's bo
 
 ```json
 {
-  "assistant": {
+  "agent": {
     "version": "2",
     "default_model": {
       "provider": "zed.dev",
@@ -484,7 +484,7 @@ Example configuration:
 
 ```json
 {
-  "assistant": {
+  "agent": {
     "version": "2",
     "default_model": {
       "provider": "zed.dev",
@@ -517,7 +517,7 @@ One with Claude 3.7 Sonnet, and one with GPT-4o.
 
 ```json
 {
-  "assistant": {
+  "agent": {
     "default_model": {
       "provider": "zed.dev",
       "model": "claude-3-7-sonnet"
@@ -532,3 +532,16 @@ One with Claude 3.7 Sonnet, and one with GPT-4o.
   }
 }
 ```
+
+## Default View
+
+Use the `default_view` setting to set change the default view of the Agent Panel.
+You can choose between `thread` (the default) and `text_thread`:
+
+```json
+{
+  "agent": {
+    "default_view": "text_thread".
+  }
+}
+```

docs/src/configuring-zed.md

@@ -3284,6 +3284,7 @@ Run the `theme selector: toggle` action in the command palette to see a current
   "dock": "right",
   "default_width": 640,
   "default_height": 320,
+  "default_view": "thread",
   "default_model": {
     "provider": "zed.dev",
     "model": "claude-3-7-sonnet-latest"