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/config/src/config_toml.rs

@@ -146,7 +146,7 @@ pub struct ConfigToml {
     #[serde(default)]
     pub permissions: Option<PermissionsToml>,
 
-    /// Deprecated optional external command to spawn for end-user notifications.
+    /// Optional external command to spawn for end-user notifications.
     #[serde(default)]
     pub notify: Option<Vec<String>>,