deprecate legacy notify (#20524)

# Why

`notify` is the remaining compatibility surface from the legacy hook
implementation. The newer lifecycle hook engine now owns the active hook
system, so we should start steering users away from adding new `notify`
configs before removing the old path entirely. This also adds a
lightweight watchpoint for the deprecation so we can see how much legacy
usage remains before the clean drop.

# What

- emit a startup deprecation notice when a non-empty `notify` command is
configured
- emit `codex.notify.configured` when a session starts with legacy
`notify` configured
- emit `codex.notify.run` when the legacy notify path fires after a
completed turn
- mark `notify` as deprecated in the config schema and repo docs
- remove the orphaned `codex-rs/hooks/src/user_notification.rs` file
that is no longer compiled
- add regression coverage for the new deprecation notice

# Next steps

A follow-up PR can remove the legacy notify path entirely once we are
ready for the clean drop. Before then, we can watch
`codex.notify.configured` and `codex.notify.run` to understand the
deprecation impact and remaining active usage. The cleanup PR should
then delete the `notify` config field, the `legacy_notify`
implementation, the old compatibility dispatch types and callsites that
only exist for the legacy path, and the remaining compatibility
docs/tests.

# Testing

- `cargo test -p codex-hooks`
- `cargo test -p codex-config`
- `cargo test -p codex-core emits_deprecation_notice_for_notify`

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

@@ -479,7 +479,7 @@ pub struct Config {
     /// - `Some("...")`: use the provided attribution text verbatim
     pub commit_attribution: Option<String>,
 
-    /// Optional external notifier command. When set, Codex will spawn this
+    /// Deprecated 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
@@ -498,7 +498,7 @@ pub struct Config {
     /// notify-send Codex '{"type":"agent-turn-complete","turn-id":"12345"}'
     /// ```
     ///
-    /// If unset the feature is disabled.
+    /// If unset the feature is disabled. Use lifecycle hooks for new automation.
     pub notify: Option<Vec<String>>,
 
     /// TUI notification settings, including enabled events, delivery method, and focus condition.