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/thread-manager-sample/src/main.rs

@@ -136,6 +136,7 @@ async fn run_main(arg0_paths: Arg0DispatchPaths) -> anyhow::Result<()> {
         state_db,
         installation_id,
         /*attestation_provider*/ None,
+        /*external_time_provider*/ None,
     );
 
     let NewThread {