Custom prompts begin with /prompts: (#4476)

<img width="608" height="354" alt="Screenshot 2025-09-29 at 4 41 08 PM"
src="https://github.com/user-attachments/assets/162508eb-c1ac-4bc0-95f2-5e23cb4ae428"
/>

codex-rs/protocol/src/custom_prompts.rs

@@ -3,6 +3,12 @@ use serde::Serialize;
 use std::path::PathBuf;
 use ts_rs::TS;
 
+/// Base namespace for custom prompt slash commands (without trailing colon).
+/// Example usage forms constructed in code:
+/// - Command token after '/': `"{PROMPTS_CMD_PREFIX}:name"`
+/// - Full slash prefix: `"/{PROMPTS_CMD_PREFIX}:"`
+pub const PROMPTS_CMD_PREFIX: &str = "prompts";
+
 #[derive(Serialize, Deserialize, Debug, Clone, TS)]
 pub struct CustomPrompt {
     pub name: String,