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/m_2025_05_05/settings.rs

@@ -0,0 +1,41 @@
+use std::ops::Range;
+use tree_sitter::{Query, QueryMatch};
+
+use crate::{
+    MigrationPatterns, patterns::SETTINGS_ASSISTANT_PATTERN,
+    patterns::SETTINGS_EDIT_PREDICTIONS_ASSISTANT_PATTERN,
+};
+
+pub const SETTINGS_PATTERNS: MigrationPatterns = &[
+    (SETTINGS_ASSISTANT_PATTERN, rename_assistant),
+    (
+        SETTINGS_EDIT_PREDICTIONS_ASSISTANT_PATTERN,
+        rename_edit_prediction_assistant,
+    ),
+];
+
+fn rename_assistant(
+    _contents: &str,
+    mat: &QueryMatch,
+    query: &Query,
+) -> Option<(Range<usize>, String)> {
+    let key_capture_ix = query.capture_index_for_name("key")?;
+    let key_range = mat
+        .nodes_for_capture_index(key_capture_ix)
+        .next()?
+        .byte_range();
+    return Some((key_range, "agent".to_string()));
+}
+
+fn rename_edit_prediction_assistant(
+    _contents: &str,
+    mat: &QueryMatch,
+    query: &Query,
+) -> Option<(Range<usize>, String)> {
+    let key_capture_ix = query.capture_index_for_name("enabled_in_assistant")?;
+    let key_range = mat
+        .nodes_for_capture_index(key_capture_ix)
+        .next()?
+        .byte_range();
+    return Some((key_range, "enabled_in_text_threads".to_string()));
+}