Add Config for Time Reminders (varlatency 1/n) (#28822)

## Summary

Example:

> [features.current_time_reminder]
enabled = true
reminder_interval_model_requests = 1
clock_source = "system"

## Testing

- `just test -p codex-core varlatency`
- `just test -p codex-core
lock_contains_prompts_and_materializes_features`
- `just fix -p codex-core -p codex-config -p codex-features`

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

@@ -278,6 +278,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         ghost_snapshot: GhostSnapshotConfig::default(),
         multi_agent_v2: MultiAgentV2Config::default(),
         rollout_budget: None,
+        current_time_reminder: None,
         features: Default::default(),
         suppress_unstable_features_warning: false,
         active_project: ProjectConfig { trust_level: None },