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/lib.rs

@@ -1,4 +1,5 @@
 pub mod config_types;
+pub mod custom_prompts;
 pub mod mcp_protocol;
 pub mod message_history;
 pub mod models;