Remove remaining custom prompt support (#16115)

## Summary
- remove protocol and core support for discovering and listing custom
prompts
- simplify the TUI slash-command flow and command popup to built-in
commands only
- delete obsolete custom prompt tests, helpers, and docs references
- clean up downstream event handling for the removed protocol events

codex-rs/rollout/src/policy.rs

@@ -161,7 +161,6 @@ fn event_msg_persistence_mode(ev: &EventMsg) -> Option<EventPersistenceMode> {
         | EventMsg::McpListToolsResponse(_)
         | EventMsg::McpStartupUpdate(_)
         | EventMsg::McpStartupComplete(_)
-        | EventMsg::ListCustomPromptsResponse(_)
         | EventMsg::ListSkillsResponse(_)
         | EventMsg::PlanUpdate(_)
         | EventMsg::ShutdownComplete