[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/app-server/tests/suite/v2/sleep.rs

@@ -167,7 +167,8 @@ wire_api = "responses"
 request_max_retries = 0
 stream_max_retries = 0
 
-[features]
+[features.current_time_reminder]
+enabled = true
 sleep_tool = true
 "#
         ),