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/protocol/src/protocol.rs

@@ -10,6 +10,7 @@ use std::path::PathBuf;
 use std::str::FromStr;
 use std::time::Duration;
 
+use crate::custom_prompts::CustomPrompt;
 use mcp_types::CallToolResult;
 use mcp_types::Tool as McpTool;
 use serde::Deserialize;
@@ -146,6 +147,9 @@ pub enum Op {
     /// Reply is delivered via `EventMsg::McpListToolsResponse`.
     ListMcpTools,
 
+    /// Request the list of available custom prompts.
+    ListCustomPrompts,
+
     /// Request the agent to summarize the current conversation context.
     /// The agent will use its existing context (either conversation history or previous response id)
     /// to generate a summary which will be returned as an AgentMessage event.
@@ -472,6 +476,9 @@ pub enum EventMsg {
     /// List of MCP tools available to the agent.
     McpListToolsResponse(McpListToolsResponseEvent),
 
+    /// List of custom prompts available to the agent.
+    ListCustomPromptsResponse(ListCustomPromptsResponseEvent),
+
     PlanUpdate(UpdatePlanArgs),
 
     TurnAborted(TurnAbortedEvent),
@@ -806,6 +813,12 @@ pub struct McpListToolsResponseEvent {
     pub tools: std::collections::HashMap<String, McpTool>,
 }
 
+/// Response payload for `Op::ListCustomPrompts`.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct ListCustomPromptsResponseEvent {
+    pub custom_prompts: Vec<CustomPrompt>,
+}
+
 #[derive(Debug, Default, Clone, Deserialize, Serialize)]
 pub struct SessionConfiguredEvent {
     /// Unique id for this session.