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/exec/src/event_processor_with_human_output.rs

@@ -533,6 +533,9 @@ impl EventProcessor for EventProcessorWithHumanOutput {
             EventMsg::McpListToolsResponse(_) => {
                 // Currently ignored in exec output.
             }
+            EventMsg::ListCustomPromptsResponse(_) => {
+                // Currently ignored in exec output.
+            }
             EventMsg::TurnAborted(abort_reason) => match abort_reason.reason {
                 TurnAbortReason::Interrupted => {
                     ts_println!(self, "task interrupted");