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/core/src/codex.rs

@@ -4385,10 +4385,6 @@ async fn submission_loop(sess: Arc<Session>, config: Arc<Config>, rx_sub: Receiv
                     handlers::reload_user_config(&sess).await;
                     false
                 }
-                Op::ListCustomPrompts => {
-                    handlers::list_custom_prompts(&sess, sub.id.clone()).await;
-                    false
-                }
                 Op::ListSkills { cwds, force_reload } => {
                     handlers::list_skills(&sess, sub.id.clone(), cwds, force_reload).await;
                     false
@@ -4514,13 +4510,11 @@ mod handlers {
     use crate::tasks::UserShellCommandMode;
     use crate::tasks::UserShellCommandTask;
     use crate::tasks::execute_user_shell_command;
-    use codex_protocol::custom_prompts::CustomPrompt;
     use codex_protocol::protocol::CodexErrorInfo;
     use codex_protocol::protocol::ErrorEvent;
     use codex_protocol::protocol::Event;
     use codex_protocol::protocol::EventMsg;
     use codex_protocol::protocol::InterAgentCommunication;
-    use codex_protocol::protocol::ListCustomPromptsResponseEvent;
     use codex_protocol::protocol::ListSkillsResponseEvent;
     use codex_protocol::protocol::McpServerRefreshConfig;
     use codex_protocol::protocol::Op;
@@ -4907,23 +4901,6 @@ mod handlers {
         sess.send_event_raw(event).await;
     }
 
-    pub async fn list_custom_prompts(sess: &Session, sub_id: String) {
-        let custom_prompts: Vec<CustomPrompt> =
-            if let Some(dir) = crate::custom_prompts::default_prompts_dir() {
-                crate::custom_prompts::discover_prompts_in(&dir).await
-            } else {
-                Vec::new()
-            };
-
-        let event = Event {
-            id: sub_id,
-            msg: EventMsg::ListCustomPromptsResponse(ListCustomPromptsResponseEvent {
-                custom_prompts,
-            }),
-        };
-        sess.send_event_raw(event).await;
-    }
-
     pub async fn list_skills(
         sess: &Session,
         sub_id: String,
@@ -6845,7 +6822,6 @@ fn realtime_text_for_event(msg: &EventMsg) -> Option<String> {
         | EventMsg::TurnDiff(_)
         | EventMsg::GetHistoryEntryResponse(_)
         | EventMsg::McpListToolsResponse(_)
-        | EventMsg::ListCustomPromptsResponse(_)
         | EventMsg::ListSkillsResponse(_)
         | EventMsg::SkillsUpdateAvailable
         | EventMsg::PlanUpdate(_)