revert legacy notify deprecation (#21152)

# Why

Revert #20524 for now because the computer use plugin has not migrated
off legacy `notify` yet. Keeping the deprecation in place today would
show users a warning before the plugin path is ready to move, so this
rolls the change back until that migration is complete.

# What

- revert the legacy `notify` deprecation change from #20524
- restore the prior `notify` behavior and remove the temporary
deprecation metrics/docs from that change

Once the computer use plugin has migrated, we can land the same
deprecation again.

codex-rs/core/src/config/mod.rs

@@ -480,7 +480,7 @@ pub struct Config {
     /// - `Some("...")`: use the provided attribution text verbatim
     pub commit_attribution: Option<String>,
 
-    /// Deprecated optional external notifier command. When set, Codex will spawn this
+    /// Optional external notifier command. When set, Codex will spawn this
     /// program after each completed *turn* (i.e. when the agent finishes
     /// processing a user submission). The value must be the full command
     /// broken into argv tokens **without** the trailing JSON argument - Codex
@@ -499,7 +499,7 @@ pub struct Config {
     /// notify-send Codex '{"type":"agent-turn-complete","turn-id":"12345"}'
     /// ```
     ///
-    /// If unset the feature is disabled. Use lifecycle hooks for new automation.
+    /// If unset the feature is disabled.
     pub notify: Option<Vec<String>>,
 
     /// TUI notification settings, including enabled events, delivery method, and focus condition.