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/migrator/src/migrations.rs

@@ -57,3 +57,9 @@ pub(crate) mod m_2025_04_23 {
 
     pub(crate) use settings::SETTINGS_PATTERNS;
 }
+
+pub(crate) mod m_2025_05_05 {
+    mod settings;
+
+    pub(crate) use settings::SETTINGS_PATTERNS;
+}