## Summary
- add a cancellable sleep operation to `TimeProvider`
- route `clock.sleep` through the configured provider
- extend the supported sleep duration to 12 hours
- complete the sleep turn item before propagating provider failures
## Why
This isolates the core clock abstraction needed by external clock
integrations. Existing system and app-server behavior remains wall-clock
based in this PR; the stacked follow-up supplies app-server sleeps from
an external clock.
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`