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>

assets/keymaps/default-linux.json

@@ -220,9 +220,9 @@
       "ctrl-shift-e": "project_panel::ToggleFocus",
       "ctrl-g": "search::SelectNextMatch",
       "ctrl-shift-g": "search::SelectPreviousMatch",
-      "ctrl-alt-/": "assistant::ToggleModelSelector",
+      "ctrl-alt-/": "agent::ToggleModelSelector",
       "ctrl-k h": "assistant::DeployHistory",
-      "ctrl-k l": "assistant::OpenRulesLibrary",
+      "ctrl-k l": "agent::OpenRulesLibrary",
       "new": "assistant::NewChat",
       "ctrl-t": "assistant::NewChat",
       "ctrl-n": "assistant::NewChat"
@@ -237,7 +237,7 @@
       "save": "workspace::Save",
       "ctrl->": "assistant::QuoteSelection",
       "ctrl-<": "assistant::InsertIntoEditor",
-      "ctrl-alt-/": "assistant::ToggleModelSelector",
+      "ctrl-alt-/": "agent::ToggleModelSelector",
       "shift-enter": "assistant::Split",
       "ctrl-r": "assistant::CycleMessageRole",
       "enter": "assistant::ConfirmCommand",
@@ -251,9 +251,9 @@
       "ctrl-alt-n": "agent::NewTextThread",
       "ctrl-shift-h": "agent::OpenHistory",
       "ctrl-alt-c": "agent::OpenConfiguration",
-      "ctrl-alt-p": "assistant::OpenRulesLibrary",
+      "ctrl-alt-p": "agent::OpenRulesLibrary",
       "ctrl-i": "agent::ToggleProfileSelector",
-      "ctrl-alt-/": "assistant::ToggleModelSelector",
+      "ctrl-alt-/": "agent::ToggleModelSelector",
       "ctrl-shift-a": "agent::ToggleContextPicker",
       "ctrl-shift-o": "agent::ToggleNavigationMenu",
       "ctrl-shift-i": "agent::ToggleOptionsMenu",
@@ -568,7 +568,7 @@
       "ctrl-shift-e": "project_panel::ToggleFocus",
       "ctrl-shift-b": "outline_panel::ToggleFocus",
       "ctrl-shift-g": "git_panel::ToggleFocus",
-      "ctrl-?": "assistant::ToggleFocus",
+      "ctrl-?": "agent::ToggleFocus",
       "alt-save": "workspace::SaveAll",
       "ctrl-alt-s": "workspace::SaveAll",
       "ctrl-k m": "language_selector::Toggle",