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/protocol/src/lib.rs

@@ -5,7 +5,6 @@ pub use agent_path::AgentPath;
 pub use thread_id::ThreadId;
 pub mod approvals;
 pub mod config_types;
-pub mod custom_prompts;
 pub mod dynamic_tools;
 pub mod items;
 pub mod mcp;