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/mcp-server/src/codex_tool_runner.rs

@@ -336,7 +336,6 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::McpToolCallBegin(_)
                     | EventMsg::McpToolCallEnd(_)
                     | EventMsg::McpListToolsResponse(_)
-                    | EventMsg::ListCustomPromptsResponse(_)
                     | EventMsg::ListSkillsResponse(_)
                     | EventMsg::ExecCommandBegin(_)
                     | EventMsg::TerminalInteraction(_)