Update assistant to agent in settings and keymaps (#29943)

Closes #ISSUE

Release Notes:

- Agent Beta: Renamed the top-level `assistant` settings key to `agent`.
A migration for existing settings files is included.
- Agent Beta: Moved the `assistant::ToggleFocus`,
`assistant::ToggleModelSelector`, and `assistant::OpenRulesLibrary`
actions to the `agent` namespace. Existing keymaps that mention these
actions by their old names will continue to work.

---------

Co-authored-by: Max <max@zed.dev>

crates/agent/src/thread.rs

@@ -998,7 +998,7 @@ impl Thread {
         for message in &self.messages {
             text.push_str(match message.role {
                 language_model::Role::User => "User:",
-                language_model::Role::Assistant => "Assistant:",
+                language_model::Role::Assistant => "Agent:",
                 language_model::Role::System => "System:",
             });
             text.push('\n');
@@ -2360,7 +2360,7 @@ impl Thread {
                 "## {role}\n",
                 role = match message.role {
                     Role::User => "User",
-                    Role::Assistant => "Assistant",
+                    Role::Assistant => "Agent",
                     Role::System => "System",
                 }
             )?;