Custom /prompts (#2696)

Adds custom `/prompts` to `~/.codex/prompts/<command>.md`.

<img width="239" height="107" alt="Screenshot 2025-08-25 at 6 22 42 PM"
src="https://github.com/user-attachments/assets/fe6ebbaa-1bf6-49d3-95f9-fdc53b752679"
/>

---

Details:

1. Adds `Op::ListCustomPrompts` to core.
2. Returns `ListCustomPromptsResponse` with list of `CustomPrompt`
(name, content).
3. TUI calls the operation on load, and populates the custom prompts
(excluding prompts that collide with builtins).
4. Selecting the custom prompt automatically sends the prompt to the
agent.

codex-rs/tui/src/chatwidget.rs

@@ -19,6 +19,7 @@ use codex_core::protocol::ExecApprovalRequestEvent;
 use codex_core::protocol::ExecCommandBeginEvent;
 use codex_core::protocol::ExecCommandEndEvent;
 use codex_core::protocol::InputItem;
+use codex_core::protocol::ListCustomPromptsResponseEvent;
 use codex_core::protocol::McpListToolsResponseEvent;
 use codex_core::protocol::McpToolCallBeginEvent;
 use codex_core::protocol::McpToolCallEndEvent;
@@ -153,6 +154,8 @@ impl ChatWidget {
             event,
             self.show_welcome_banner,
         ));
+        // Ask codex-core to enumerate custom prompts for this session.
+        self.submit_op(Op::ListCustomPrompts);
         if let Some(user_message) = self.initial_user_message.take() {
             self.submit_user_message(user_message);
         }
@@ -991,6 +994,7 @@ impl ChatWidget {
             EventMsg::WebSearchBegin(ev) => self.on_web_search_begin(ev),
             EventMsg::GetHistoryEntryResponse(ev) => self.on_get_history_entry_response(ev),
             EventMsg::McpListToolsResponse(ev) => self.on_list_mcp_tools(ev),
+            EventMsg::ListCustomPromptsResponse(ev) => self.on_list_custom_prompts(ev),
             EventMsg::ShutdownComplete => self.on_shutdown_complete(),
             EventMsg::TurnDiff(TurnDiffEvent { unified_diff }) => self.on_turn_diff(unified_diff),
             EventMsg::BackgroundEvent(BackgroundEventEvent { message }) => {
@@ -1220,6 +1224,13 @@ impl ChatWidget {
         self.add_to_history(history_cell::new_mcp_tools_output(&self.config, ev.tools));
     }
 
+    fn on_list_custom_prompts(&mut self, ev: ListCustomPromptsResponseEvent) {
+        let len = ev.custom_prompts.len();
+        debug!("received {len} custom prompts");
+        // Forward to bottom pane so the slash popup can show them now.
+        self.bottom_pane.set_custom_prompts(ev.custom_prompts);
+    }
+
     /// Programmatically submit a user text message as if typed in the
     /// composer. The text will be added to conversation history and sent to
     /// the agent.