[codex] nest sleep config under current time reminder (#29910)

## Summary

- move sleep tool enablement from top-level `[features].sleep_tool` to
`[features.current_time_reminder].sleep_tool`
- remove the standalone `Feature::SleepTool` flag and gate `clock.sleep`
from resolved current-time configuration
- update config schema, config-lock materialization, and existing sleep
coverage

Stacked on #29907.

codex-rs/core/tests/suite/code_mode.rs

@@ -883,6 +883,7 @@ text(JSON.stringify(result));
             config.current_time_reminder = Some(CurrentTimeReminderConfig {
                 reminder_interval_seconds: 3_000,
                 clock_source: CurrentTimeSource::System,
+                ..CurrentTimeReminderConfig::default()
             });
         },
     )

codex-rs/core/tests/suite/current_time_reminder.rs

@@ -86,6 +86,7 @@ fn enable_current_time_reminder(
     config.current_time_reminder = Some(CurrentTimeReminderConfig {
         reminder_interval_seconds: interval,
         clock_source,
+        ..CurrentTimeReminderConfig::default()
     });
 }
 

codex-rs/core/tests/suite/pending_input.rs

@@ -2,6 +2,7 @@ use core_test_support::test_codex::local_selections;
 use std::sync::Arc;
 
 use codex_core::CodexThread;
+use codex_core::config::CurrentTimeReminderConfig;
 use codex_features::Feature;
 use codex_protocol::AgentPath;
 use codex_protocol::items::SleepItem;
@@ -390,8 +391,12 @@ async fn any_new_input_interrupts_sleep() {
         .with_config(|config| {
             config
                 .features
-                .enable(Feature::SleepTool)
-                .expect("test config should allow feature update");
+                .enable(Feature::CurrentTimeReminder)
+                .expect("test config should allow current-time reminders");
+            config.current_time_reminder = Some(CurrentTimeReminderConfig {
+                sleep_tool: true,
+                ..CurrentTimeReminderConfig::default()
+            });
         })
         .build_with_streaming_server(&server)
         .await