[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/src/session/config_lock.rs

@@ -359,6 +359,7 @@ mod tests {
                 enabled: Some(true),
                 reminder_interval_seconds: Some(1),
                 clock_source: Some(codex_features::CurrentTimeSource::System),
+                sleep_tool: Some(false),
             }))
         );