current time reminders impl for system clock (varlatency 2/n) (#28824)

Stacked on #28822.

## Summary

- add a host-injectable current-time provider with a built-in system
implementation
- record UTC developer reminders in history immediately before due model
requests
- keep cadence state per session and force a refresh after compaction

This does NOT include the app server client <-> server clock logic. This
PR is only for the reminder message & system clock that will be used in
prod.

## Testing

- `just test -p codex-core varlatency_`
- `just clippy -p codex-core -p codex-app-server -p codex-mcp-server -p
codex-thread-manager-sample`
- `just fmt`

codex-rs/app-server/src/message_processor.rs

@@ -379,6 +379,7 @@ impl MessageProcessor {
                     outgoing.clone(),
                     thread_state_manager.clone(),
                 )),
+                /*external_time_provider*/ None,
             )
         });
         let models_manager = thread_manager.get_models_manager();