[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()
             });
         },
     )