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

crates/agent/src/agent_panel.rs

@@ -525,8 +525,8 @@ impl AgentPanel {
 
         let panel_type = AssistantSettings::get_global(cx).default_view;
         let active_view = match panel_type {
-            DefaultView::Agent => ActiveView::thread(thread.clone(), window, cx),
-            DefaultView::Thread => {
+            DefaultView::Thread => ActiveView::thread(thread.clone(), window, cx),
+            DefaultView::TextThread => {
                 let context =
                     context_store.update(cx, |context_store, cx| context_store.create(cx));
                 let lsp_adapter_delegate = make_lsp_adapter_delegate(&project.clone(), cx).unwrap();

crates/assistant_settings/src/assistant_settings.rs

@@ -35,8 +35,8 @@ pub enum AssistantDockPosition {
 #[serde(rename_all = "snake_case")]
 pub enum DefaultView {
     #[default]
-    Agent,
     Thread,
+    TextThread,
 }
 
 #[derive(Copy, Clone, Default, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
@@ -614,19 +614,19 @@ pub struct AssistantSettingsContentV2 {
     ///
     /// Default: true
     enabled: Option<bool>,
-    /// Whether to show the assistant panel button in the status bar.
+    /// Whether to show the agent panel button in the status bar.
     ///
     /// Default: true
     button: Option<bool>,
-    /// Where to dock the assistant.
+    /// Where to dock the agent panel.
     ///
     /// Default: right
     dock: Option<AssistantDockPosition>,
-    /// Default width in pixels when the assistant is docked to the left or right.
+    /// Default width in pixels when the agent panel is docked to the left or right.
     ///
     /// Default: 640
     default_width: Option<f32>,
-    /// Default height in pixels when the assistant is docked to the bottom.
+    /// Default height in pixels when the agent panel is docked to the bottom.
     ///
     /// Default: 320
     default_height: Option<f32>,
@@ -644,9 +644,9 @@ pub struct AssistantSettingsContentV2 {
     ///
     /// Default: write
     default_profile: Option<AgentProfileId>,
-    /// The default assistant panel type.
+    /// Which view type to show by default in the agent panel.
     ///
-    /// Default: agentic
+    /// Default: "thread"
     default_view: Option<DefaultView>,
     /// The available agent profiles.
     pub profiles: Option<IndexMap<AgentProfileId, AgentProfileContent>>,
@@ -676,7 +676,6 @@ pub struct AssistantSettingsContentV2 {
     /// Default: []
     #[serde(default)]
     model_parameters: Vec<LanguageModelParameters>,
-
     /// What completion mode to enable for new threads
     ///
     /// Default: normal